I just finished mounting a scope & sighting it in for a friend's Gamo Swarm Maxxim that has similar specs to your rifle. It came with a 3-9 mounted. I removed the base & rings, cleaned & degreased, then remounted using Loctite.
It wasn't grouping worth a darn either using match pellets that my RWS 36 loves. I switched to the Gamo Whisper pellets & immediately saw my groups shrink. These are longer & heavier (10.5 grains) & Gamo recommends them for their rifles that use the Whisper technology.
These stay subsonic, so that helps accuracy. They also don't generate the sonic crack that normal weight pellets do at the guns rated 1,300 fps, so they are also much quieter.
This might sound odd but how you hold it might make a difference. I get my best results by resting the forearm on my open palm and allowing the stock to be loose against my shoulder. When I try to hold tight my groups open up. it might have something to do with the oddball reverse recoil.