The Sprint TP2 is a world phone. It does have a GSM sim as well. If you buy the Sprint phone, you need to make sure you only put CDMA ROMS on it. The links above should also tell you how to use the phone in GSM only.
The Tmobile or AT&T versions are GSM phones. GSM roms are found over at
http://www.xda-developers.com/. You need to make sure you only put GSM roms on a GSM version of the phone.