How do I get my GPS working on iPhone 3GS after upgrading to iOS 5? I've upgraded my iPhone 3GS to iOS 5 and hence changed my baseband to 5.16.05. Now it is not possible for me to unlock the phone. Is there any way I could get both GPS and phone working?. I've done jailbreak with redsnow. I don't need iOS 5 and is willing to downgrade without breaking anything. Is it possible to do it?
No. You are out of luck.

Even if you had saved the SHSH blobs for iOS 4.X (which are what you'll require to rollback), you can't revert the baseband, which is required for the unlock. So even if you did manage to install iOS 4.X, you'd still be running baseband 5.16.05.