You just need to do the following steps to get back GPS on your T-Mobile GS3
1. Install TriangleAway
here
2. Open TriangleAway, reset flashing counter (you must have root access)
3. Download original firmware for T-Mobile Galaxy S3 (T999UVALEM)
here. Thanks to SamMobile
4. Boot your phone into Download Mode (Volume down + power + home) and flash the firmware above via ODIN
5. If you have bootloop after flashing, remove battery, boot your phone into Recovery Mode, wipe data (factory reset) + wipe cache
6. When your phone finishes rebooting, you will see the GPS has been fixed and worked perfectly.
7. Upgrading to the latest JB 4.1.1 by flashing the rooted firmware by DocHoliday77
here. GPS still works.