Humm it should work even http://www.t-mobile.com/optional-services/roaming.html when I entered the SCH-I545 which is the Verizon model it said it should work and it looks that the phone supports HSPA 3g and GSM 2g on the following frequencies HSPA+/UMTS: 850/900/1900/2100MHz; GSM: 850/900/1800/1900MHz
Do you have the option to lock the phone into GSM mode or does the menu only have "global". If you go into networks and search does anything come up and/or put it in manual mode.
You might want to call T-Mobile and see if they can verify your plan is set up correctly, I know the international works because I use it all the time as do my friends one of which uses it in Thailand, Malaysia and Indonesia all the time since he is a contract pilot over there.