Gamos are OK guns but for true quality, buy a(in order of increasing amt. of workmanship) Beeman, RWS, or a Weihrauch. These guns will consistently put down anything smaller than a coyote, and if you do want to take airgun yotes, buy an Airforce Airguns Talon, Talon SS,or Condor and shoot Beeman Kodiak orextra-heavy Eun Jin pellets. Condor'smuzzle energyis about the same as that of standard .22 lr loads.
Best airgun supplier: pyramydair.com. If you want to take large game google axiomlemak and look for the talon airbolt mod. Or just buy a large-bore gun from pyramyadair.com or look for quackenbush airguns. But for the best guns ever built, Webley Tomahawk or Patriot.
Yes I have done a lot of research on airguns; I desperately wanted a Talon SS before I found that airgun hunting is not legal in Florida.

Edit: For yotes w/ the aforementioned guns you need head shots in .22 or .25cal!!!! Body will put it down but wont kill it until it's hit again in the head.

Edit Edit: Never use PBA pellets on a spring piston gun!!! Ever!!! Gamo only uses them to get their velocity ratings up. These pellets are lighter which means that they travel faster. At a glance that seems like a great thing, but these pellets exit the barrels long before the piston hits the front of the compression chamber so there is no leftover pressure to cushion it. It is the equivalent of dry-firing a bow. It is very damaging to your gun. However, pumps and pre-charged pneumatics(PCPs) can shoot PBA w/o any adverse effects b/c they just release compressed gas.