You can get banned for any sort of modding, especially modding a .gpd.
If you add a little bit of gamerscore at a time, and time stamp all of them, your chances of being banned is low. Be wary of ones you can only get by playing online.
Also, after modding anything in a gpd, load your profile to xbox live for approximately 5 minutes then delete and redownload your profile. If anything doesn't stick, that's fine, just add the achievements you missed.
Yes you can be banned. Make sure you take precautions as iThrowAIDS
said. Be careful as you continue modding your score though. Don't want to end up like me having to wait for my account.