A few thoughts on this after having one of these buggers apart. If you can get the grub screw on the bottom of the cylinder out, the barrel SHOULD unscrew. It is a left handed thread. Deejayuu managed to drill out the damaged grub screws and unscrew the barrels on 2 actions here but he said one was a swine. Anyhow, once you unscrew the barrel, you can have a look into the breech area. IF you wanted to tinker with this, my feelings are you could fit O rings, which I would prefer to the tube sort of seal in the Gamo Falcon/Center. On the cylinder side a small hole could be drilled and a small O ring fitted round the transfer port. On the barrel side, My feelings are rather than shortening the barrel and fitting an O ring to make up the difference, turn a thread or 2 down on the outside of the barrel and fit an O ring on that.
I tried to Pm yesterday, dropped the mouse and lost the mesage
Hopefully, soon, there will be some bits to enable you to tinker, on the way to you.
Watch out for somebody fitting a .22 tap to a .177 barrel and a .177 tap to a .22 barrel.