I have been thinking about getting a lathe. Landed on the Grizzly g4003g. My interests are truing Remington 700 actions and threading barrels, maybe chambering in the future.

What other tooling is a "must have" to perform this work?

I have a few 0.001" dial indicators but I am guessing in will need a 0.0005" and 0.0001" and magnetic bases to get everything set up in the jaws.

I'm trying to learn before I start purchasing.

I have that exact model (but unfortunately it is still sitting outside under a tarp as the movers were unable to get it into my basement).

I'm not a gunsmith and have not done the type of work that you want to do, so I'll just watch and see what others recommend.

For example, I'm not sure how the included quick-change toolpost compares with some of the ones on the market, and I'm not sure how the chucks that come with the machine compare to others that are available, nor do I know whether a 5C collet chuck would have any value for the types of work that you'd do.

Work off of your 4 jaw chuck, the 3 jaw regardless of how many times you dial it in will not be accurate enough (IMO).
Make a set of "spiders" for holding barrels. I used an old faceplate and made my inboard spider for my spindle side and gained about 5".
Make a precision bearing "Cathead". Very handy when working barrels too short to pass through your headstock.
Indicator rods for dialing in barrels, gage pins will work as well but the longer the indicator rod, the more accurate you will be.
Learn how to single point thread correctly.
.0005 indicators will be plenty accurate, just indicate items in to zero movement of your needle.
Use your .001 indicator to 'rough it in" then do your final dial in with the .0005. Until you become practiced in dialing an item in you will drive yourself nuts "chasing the needle" with a .0005.

Get at least 5-6 tool holders including a dedicated cut off tool. Learn to grind your tools.
A quality live center is mandatory, spend the money for a Bison.

When you get your lathe change the oil in it at least 2-3 times before you really start turning with it. Flush it out well. I have seen numerous accounts of the green machines with all kinds of trash in the gearboxes from the factory, some even with sand from the casting process. Not really what you want mingling with those shiny new gears and bearings.
Clean all the cosmolene crap off really well, make sure your gibs and backlash are adjusted properly from the git go.
Clean and lube your chuck.

DO NOT REMOVE YOUR GAP BED, EVER! It will never go back into the exact same place and it can really make life miserable.

Another thought- Save up the extra coin for this one: http://www.grizzly.com/products/14-X-40-Gunsmithing-Gearhead-Lathe/G0709
The gearhead will make life easier, changing gears can be a PITA. The additional rigidity of the larger machine will be helpful as well.
Better resale too.

Is this the machine?
https://gulfport.craigslist.org/tls/d/grizzly-g4003g-gunsmithing/6510989942.html
If so it is older and some parts may be difficult to get.

Besides, he is asking more than a new machine would cost.

The tooling will be at least 50% of the cost of the base lathe.

I figured I would have to spend an additional $2-4k on tooling, and maybe have to make some of it myself. The question is: what would I need to buy for the type of work I want to do?

Thank you Dano and hockysew for the information.

Your welcome and good luck with your new venture

Just picked up a used G4003G myself that's currently sitting in my garage until I can shoehorn it into my basement shop.

That doesn't seem like that good of a deal. Hell, you could get a brand new machine for $4k delivered.

I just bought the same machine with a DRO and some tooling AND a geared head mill for $4250

What tooling would you suggest to start with?

$2k seems like a heck of a lot of inserts, drill bits, and reamers.

$2k for tooling won't get you far when you're first starting out.  Sure thst covers a lot of inserts, but not much else.
Basic tool holders are cheap, but you'll want more than a handful.
Boring bars
Good drill bits
Starter/center drills
Reamers
Grooving and Parting tools
End mills work in lathes too.  You don't always have to be turning the work.
Inserts for all of the above, and plenty of them.
Taps, dies and thread turning tools.  Inside and outside.  Some will work in standard insert holders, some require their own.
A good supply of HSS stock for making your own cutters
Knurling tool for making your homemade tooling more useful and professional,  works for thread protectors too.
Your money will start disappearing fast on items like Indicators, indicator holders, bases and mounts, coaxial, 10ths, dials, mics, calipers.  Don't cheap out much on those.  Drill inserts are nice to have for any jigs you make.

A decent grinder with good stones for making and sharpening your tooling too.  I use a band saw a lot.

Lots of various steel stock.  Some for parts you'll be making, some for tooling.  Make as much tools as you can, makes for good practice on the equipment and get to know the capabilities of you and your equipment.  Bronze and brass too for barrel bushings and the like.  Shim stock comes in handy, but if you're cheap, so are beer cans.

If the lathe you get doesn't have a quick change tool post, get one.  And lots of holders, or make some.  But having plenty will save you a lot of time and frustration.  If the right tool is readily available, you're more likely to use it.

Obviously clamping kits, vise and other work holdings get pricey fast, but are essential.  Keep an assortment of nuts, bolts, washers and machine screws around for this as well.

PPE too, Personal Protective Equipment.  Don't go getting tangled up in your lathe by dangling loose clothing around moving parts.  Keep your fingers out of stuff and your ears and eyes protected.

Time and patience.  Post lots of pictures of your projects too!

An Aloris dovetail tool holder and a set of tool holders to fit.

Do NOT get a 'piston' style tool holder.

The extra extension of the tool from the post only makes rigidity worse.
The expanding style pulls the tool against the tool holder.

I usually rework the mating surface slightly on both pieces to get an even tighter and flatter contact surface.

go on Ebay and do a search for lathe tools Lots ,,

i have bought several thousand dollars worth for the cheap ...

https://www.ebay.com/sch/i.html?_sacat=0&_nkw=machinist+tools+lot&_frs=1

One of the greatest 'accessories' is an old codger that drove a lathe for a living.

I used to know a few but they keep on getting old and dying.

At work we had a few retired guys that we would pay $400 an hour to come in and help with setting up new machinery.
Worth ever damn penny.

Oh boy, so many tools-

Noga base can't be understated.  Yes, others will work but they suck in comparison.

Thread wires

Mics/gages/indicators/calipers of all types and standards to check them.  A .0005" test indicator is the workhorse of the lathe.

A good stone for touching up tools.

Boring bars with insert tooling

Round tools out the wazoo- drills, reamers, taps, dies

A good keyless chuck for the tailstock.  I'm partial to Albrcht and they're readily found on eBay for a fraction of new cost.

Most important is probably practice material, 6061 or 4140 would be my choice.

One tool I love is an ER40 collet chuck on an MT3 with the drive tang. (Be aware that most are drilled and tapped for a draw bar and do not have the drive tang.) I spend the big $$$ to buy the proper tap drive ER40 collets which have a square drive at the rear.

So when I need to make more 7/8-14 custom lock nuts I can power tap without slipping or spinning in the taper. I can also use it to drive custom ground port tools and other large shank tools. (using regular ER40 collets)

One Brain dead idiot design change on the G4003G was to drill and tap the rear of the spindle to make a built in "Spider". The regular G4003/G4002 (short bed) is threaded in the spindle ID to accept the quick change 5C draw bar assembly. So to use 5C the easy way is to buy a 5C chuck and D1-5 mounting plate. Or buy the kit for the G4002 and have a custom replacement made for the part which threads into the rear of the spindle tube.

I am working on a print set for that part - but it is a very back burner project at this point.

I can not argue that the spider set, four loose threaded holes versus the standard is probably a cost reduction more than a selling point.

Well another hemorrhoid with the G4003G is they fit it with a D1-5 spindle face rather than the D1-4 of the G4002/G4003. There are all sorts of work holding options with integrated D1-4 mounting. Almost none in D1-5 and if you do find some they run about double the price of the D1-4 version.

I may be selling mine in the not too distant future.

I have arranged to buy an 1976 Okuma LK, with factory taper attachment, flood, 3 jaw, 4 jaw, faceplate, universal collet closer (uses Jacobs rubberflex style collets), steady and follower rests. Under the grime from storage its almost like new. Turning 2200rpm it is quieter than my G4003G in its lowest gear and the feed fwd/rev in neutral. The Okuma motor can do a 7hp cut for 30 minutes or run non-stop at 5hp. To prove the point we took a 1.0" cut off the OD at .0065" IPR @ 550 rpm - It just purred through it. The Okuma LK weighs in at nearly double and is significantly more stable since the chip pan is cast into the full base. Its price when new in 75-76 was about $11,000 with all the extras. I will be 2nd owner.