I bought a rosco barrel and bootleg low profile gas block (.750 journal). The gas block slides onto the barrel easily with no resistance. I can even spin the gas block on the barrel if I give it a good flick.

I know that if I tighten the set screws, it'll tighten the gas block against the barrel where the gas port is. Do I need to worry about these two items being a bad fit tolerance wise?

I would probably coat the inside of the block and the barrel journal with some Loctite 242 to try to fill in the gap.
Might not be necessary but wont hurt anything .............

If you take blue loctite (contrary to what all the full auto mag dump folks say, blue does work...but then again I don't run full auto or mag dump lol) and put a few drops right down in the holes then insert and tighten the set screws it will be very tight and require a good deal of heat to remove (the tightening of the set screws mashes the blue loctite into the void) if you are in doubt.

However, yes they will tighten up, make the seal and be all good even if loose going on. Dimples would be ideal too but not required.

The issue comes down to how tight is too tight. Most prefer the clamp style as it's been said set screws can affect accuracy. Not so much a big deal for range toys/plinking.

I just put a loose GB on a barrel and it worked just fine. I didn't use anything to seal the GB... after a few test rounds, I removed the GB and looked at the gas/burn marks and zero evidence of leakage.

As far as set screws effecting accuracy, that has not been my experience with multiple AR15 and AR10 builds... others experiences may differ.

Won't hurt anything.....unless it gets in the gas port.....and sets up.....then it might.

It isn't an issue.  I have one rifle with a GB that was like that.  Tightened the set screws and it has run flawlessly for over 1k rounds.

Set screws will not have any affect on accuracy.  Having a gas block attached to a barrel raises hell with harmonics regardless.