Wow--I guess I'll be the lone dissenter here. But then I also appear to be the only one posting who's actually shot cast bullets in a Garand.
Firstly, very few of my cast bullets fly slower than 1200 fps out of the muzzle, even in my handguns. My air-cooled wheel weight alloy bullets run great from about 1600 to 2100 fps out of several Garands and M1903 and '03A3s. Beyond 2100 or 2200 fps I need to go a little harder and water-quench the bullets, which allows me to get up to about 2400 fps before groups start to open up.
I can't speak to other gas systems, but the gas systems in my two Garands do just fine with cast bullets. When I first started shooting cast loads through them I was obsessed with pulling the gas cylinder and examining everything. In the thousands of cast bullets I've fired through my Garands, I have *never* seen anything more significant than a couple of tiny flakes of lead on the piston which readily wiped off.
The only trick with cast loads for the Garand is coming up with a load that generates enough pressure at the port to cycle the action reliably, but doesn't drive the bullet too fast. This requires powders that are normally considered far too slow for typical jacketed bullet loads--powders like 4350 and 4831. Wonderfully accurate cast bullet loads can be developed using 2400, 4227 and others, but they don't generate the port pressure to work the action, effectively turning the Garand into a straight-pull bolt action.
There is some really great info from a guy named BruceB at the Castboolits site in this thread here:
Note that the title relates to the M1A. Bruce started developing for his M1A but also included his Garand.