Does the rifle have an integrated dovetail type rail to serve as a scope mount where the rings will attach? Or is the rifle tapped for a regular mount like a Weaver?
If you have a dovetail type mount, I’d just buy a scope kit like this one. Gander and Fleet Farm carry scope kits as do most Walmarts.
http://www.pyramydair.com/s/a/Gamo_4x32_Scope_1_Tube_11mm_Dovetail_Rings/2392
I wouldn’t overthink this. Optical quality is of minimal concern at the distances and magnification you’ll be working with. I’d go with 4 power, anything more won’t be of any benefit.
With a CO2 powered weapon, your velocity is very limited. You need to get close to whack a bunny.
Grouse