Quoted: I just refinished the stock on my field grade but before I take it out for the first time I have a few issues that need to be resolved.  The first is that the entire gas cylinder assembly has fore and aft play in it, even when the screw/plug is tightened down.  The cylinder (and front sight) can also rock side to side.  I've read that peening the barrel splines will help with the side to side rocking, but what needs to be done to take out the fore and aft slop?

Slightly peening the splines on the barrel will remove the rotational play.  With the muzzle of the barrel fully supported, and using a sharp center oynch, upset a little of the metal on either side of the spline cut inwards into the cut.  Upsets, 4 per spline cut, should be in opposition with each other.  Best to upset too little and have to re-do than too much.  Have a good wooden/plastic mallet ready to drive on/off the gas cylinder.

As for the axial play, this should not happen if the all your parts, i.e., gas cyl, gas cyl lock, and gas cyl lock screw are of the correct dimensions and the screw is fully threaded in.  If your gas cyl lock screw does not fully seat, then either the gas cyl lock, the gas cyl, or both have been modified.  You will have to examine all the parts and compare them with known good ones to find out which may be off.  Replace as needed  In passing, this may also cure the rotational play issue, so make sure all your parts fit right before peening the splines.  If, as you say, this is an ODCMP rifle, contact them about the problem(s).  Their customer service is pretty good.  Find out which parts are out-of-spec (if any) first, though.   This might save you the hassle of sending the thing back.  Maybe they can just send you the correct parts.

Second, the rear sight assembly is very loose.  The aperature rocks easily from side to side, as does the windage base.  Will simply replacing the cover take care of this, or is there something else that needs to be done.

Most likely the spring steel cover is worn.  I presume you have carefully tightened the sight adjustment screws, right?  The cover can be either bent or replaced, although the new cover may need a bit of tweaking.  Sight guts should be cleaned, and white lube applied to the gear track, the slots for the aperture, and the underside of the cover where it rubs against the mechanism.  A little dab will do ya.  Don't go crazy.

Finally, according to Scott Duff's Garand Owner's Guide, with just the bolt and op rod installed in the receiver, both should slide back and forth freely when raising the muzzle 30 degrees and the rear 45 degrees.  Sliding out of battery is no problem, but going forward requires going almost vertical.  That said, I guess the op rod is going to require rework or replacement.  Any other suggestions?

Again, I would mention this to ODCMP, but I would shoot the rifle a bit before bringing this up.  Maybe some of the parts need to be "worn-in" and things will be all right after that.   A NEW op-spring would be a nice thing to install.  True for any Garand that still has a 50-yr old spring in it.

Quoted: Raf, thanks for the in-depth reply on this.  Regarding the gas cylinder, when I initially disassembled the rifle, I was surprised to find that the cylinder easily slid off by hand with practically zero force.  I just installed the gas cylinder, lock, and set screw from my (still in cosmoline, temporary safe-queen) HRA correct and it was also loose on the SA barrel, but not on the HRA barrel.   I noticed that the the portion of the barrel where the rear lug of the gas cylinder sets is worn to the point that the raised portion is hardly discernable.  That said, am I basically SOL unless I rebarrel it (in .308 maybe)?  Although I'm not thrilled with the prospect of major work, I can hardly fault the CMP for that possibility and I'm still glad to have bought it.  I just wish I could shoot it right away.

Sounds like external barrel wear is the root of your problem.  That said, maybe its worn internally as well.  If so, and the rifle has no obvious collector value, then re-barrel in the currently inexpensive .308.  Champion's Choice did mine, and there are other good vendors as well.

As for the rear sight issue, it has been cleaned, lubed and adjustment screws have been re-tightened prior to mentioning the previously described condition.  I tried re-bending the cover and while that helped some, it's still not solid like the HRA or the sight on my M1A.  Any recommendations on places to buy covers and other M1 items?  Thanks again.

Again, wear of the cover and/or the sight components themselves is the most likely cause.

Before you get too involved, I again urge you to contact ODCMP with your concerns.  Might save you some money.

As for M1/M-14 vendors, google Bill  Ricca.  I've bought stuff from him, and he is an honest man.  There are other good vendors as well, of course.