Any local paintball store should be able to fill 4500 psi cylinders these days- C02 is almost dead in the water.
I prefer springers, as it keeps the logistics of a day shooting airguns in the woods to a minimum. I usually shoot with aperture sights, so I'm not stretching the range. 12 ft lbs and decent accuracy are plenty for me.
Before you dump your Gamo you should try some heavier pellets. I like the Beeman Kodiaks, which shoot really well in my C-1.