It's pretty good. You might want to check the gaming forum instead, to see what people have to say over there, but I personally am satisfied with my 50" GWIII for gaming.
It's best on Gamecube or X-Box... PS2 games look dated on a normal TV, and worse on a larger TV, but this is really an issue with a low-resolution and jagged source, rather than the TV itself. In any case, the GWIII doesn't experience any of the dreaded "Samsung DLP Lag" which drives gamers crazy.