How do I get my GPS working on iPhone 3GS after upgrading to iOS 5?
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.