Actually, you can change the values, but it wouldn't do anything if GP is server based, which I think it is. Use something like UCE to scan for GP value, then change it, and freeze it. I did this once for another FPS, and it showed that I had 999999Mcash, but when I tried to buy something, it said "Not enough MCash". So you can change the value to make it appear as if you have <Insert Number Here>, but doesn't exactly mean that you will have that much.
/rant done.