Buy the factory unlock from ebay for a dollar and you won't be needing the gevey sim anymore.
Just pop in your T-mobile and it should work fine.
After the seller says that he unlocked the phone, you need to resintall your IOS and after the IOS update is complete, a message will says"Do you want to update your carrier setting" or something. Click yes and its unlocked for ever. 
The only bad part is that you will loose your jailbreak and get stuck with latest IOS for which jailbreaking is not available atm.