The rifle was designed to use .30M1 ammo with a 174.5 grain bullets at 2647fps. Later, the U.S. went to their M2 ammo with a 152 grain bullet at 2700fps, then the same bullet at 2800fps. The .30AP used a 168.5 grain bullet at 2775fps.
IMR4895 was the original powder, but IMR4064 seems to give better accuracy.
There is an internet myth that says any bullet greater than 180 grains will damage the op rod. I've never once, in 30 some years, seen nor heard of anybody damaging their rifle with ammo. The 220 grain Silvertips I used at one point, did nothing to mine. However, heavy bullets means an increase in the felt recoil. Not enough to worry about, but heavier felt recoil.
150 grain bullets will kill a deer. 165's will kill anything. 168 grain match grade bullets, while expensive, give outstanding accuracy once you've worked up the load with IMR4064.
Get a copy of Hatcher's Book of the Garand. About $30. More and better info than any internet site. Your local gun shop should have it or a gun show. Amazon lists it too, but used at much more money. Hatcher's Notebook is a good purchase too.