Do you have your iPhone 4 unlocked using Gevey SIM and want to upgrade to iOS 5? If so, you can unlock iPhone 4 running iOS 5 using Gevey SIM but you’ll need to follow few steps before doing that. In order to unlock iOS 5 with Gevey SIM, you’ll need to preserve your unlockable iPhone 4 baseband.
You’ll need the new Redsn0w’s “Custom IPSW” option which is included on Mac OS X only. Windows version of Redsn0w 0.9.9b5 coming soon along with new Ultrasn0w later today to unlock iOS 5 (only for current unlocked devices, no new unlock).

You’ll need to download iOS 5 from here, iTunes 10.5 from here and Redsn0w 0.9.9b5 from here. Now you’re ready, below is the guide to preserve iPhone 4 baseband.

STEP 1: Connect your iPhone 4 into you computer then open Redsn0w 0.9.9b5.

STEP 2: Now select Extras then “Custom IPSW” option so that you can create your pre-jailbroken custom firmware.

STEP 3: Redsn0w will start cooking your custom IPSW and you’ll get it in few minutes.

STEP 4: Once you get your firmware, select ‘Pwned DFU’ from RedSn0w.

STEP 5: Follow the on-screen instructions to get your iPhone in DFU mode.

STEP 6: Once it’s in DFU mode, close Redsn0w then open iTunes 10.5 downloaded above and you should get your iPhone in Recovery Mode message.

STEP 7: Now hold the Option key on Mac and hit Restore. Point iTunes to your iPhone 4 Custom iOS 5 IPSW which begins with NO_BB. (WARNING: Don’t select the stock one or you’ll lose your unlock)