If it won't cock all the way sometimes but will cock others there is probably something out of place inside. If it's really easy to cock that something could be pieces of a broken spring. At the very least you're probably looking at a new spring and probably a piston seal while you're inside. If you're up for a project the price isn't too bad. You might price a spring and use that as ammunition to have the pawn shop owner lower his price. If he has a $50 tag on the gun that's what he's hoping for. The final price is negotiable. Checking Amazon it appears Gamo rifles start at about $90 new. Fortunately they're not too hard to work on.
I'm not sure if there is a distinction to be made, but the 300 is an "ElGamo" rather than a "Gamo"