Tuning HELP! Gamo urban .22

I think Pyramid Air can work on them. It is most likely just an o-ring behind the air gauge. Mine leaked from there when I first got it and it was easy enough to take care of on my own. Let all the air out and slowly unscrew the gauge. Once it is off remove the o-ring and replace. Put a small bit of silicone grease on the ring to keep it fresh and re-install. Do NOT over tighten, a quarter turn past hand tight is all you need.