This is a post for the iPhone 4 users that have upgraded to iOS 5 firmware. The iOS 5 firmware kills the previous unlock but the Gevey SIM users can unlock their devices. To unlock your device with Gevey, you have to update the firmware to pre jailbroken firmware in order to preserve the unlocked (or unlockable) baseband that works with Gevey SIM.
To preserve the baseband, connect your iPhone via USB port and open Redsnow. You should also be running the latest version of iTunes (v10.5) and iOS 5 IPSW. All download links are at the bottom of this post.
Start redsnow; go to ‘extras’>’custom ipsw’ and browse for the firmware you downloaded. The app will analyze it. Then, click on the button that says ‘Pwned DFU’. Turn your device off and plug it in before clicking next. Your IPSW will be generated which you can flash under iTunes (select your device; press and hold the cmd button and browse for the IPSW generated by Redsnow).
Right now, this is a tethered redsn0w jailbreak which means that you must jailbreak it again after doing a soft reset.