The easiest way to unscrew a Gamo pivot bolt is to send the gun to Gamo. They don't WANT end users fixing their guns. They use some kind of locking compound on their bolts. Some users end up drilling the bolts out and replacing them. Gamo guns aren't bad but getting parts can be difficult. They generally want you to go through a federally licensed gun dealer to order them. That's why I have no Gamo guns.
The only way I have been able to remove that bolt is to drill out the thread the apply a little heat. This destroys the bolt but you can get replacement bolts..