I assume it is a nitro spring gun? Nitro springs have a high failure rate from leaking out over time, so if it's a nitro then I'd say 90% chance that's the problem. It's easy to determine by cocking effort, if it takes about 30lbs to cock it then it's probably fine, if less then it has leaked by a similar amount. Judging by the power you described I'd say it'll be less than 10lbs to cock, if that is the problem.
If that's fine then I'd check the breech seal to see if it's leaking, or possibly missing. The main seals usually leak but that much would be very abnormal. I'd consider buying new main and breech seals for it regardless. You should also check the bore for rust and transfer port for rust or obstructions. If you need to take the gun apart then I can't say not to use a spring compressor, but I will say I have never needed one.
Usually Crosman parts can be used, but Gamo parts vary here and there so I can't say for sure. I use Crosman when I can because their parts are usually much cheaper than anyone else. For example the main and breech seals are only $1.15 ea, a main coil spring is <$6, a nitro spring is $26. Shipping is a flat $4 per order. You can write me at chevota at hotmail if you have questions. I also have a nice tuning guide and trigger guide if you want to tune the gun while it's apart.
