Paul makes some good points, I think size and weight of the scope is more important on the heavy recoiling guns than AO or non-AO. One of the reasons I like the Burris Timberline 4.5-14x32. You can still get a good power range, but it's small, under 11.5", and light at only 15 oz. And because it's a 32 mm, low or medium mounts work well on most rifles, decreasing the leverage of the hard kickers. That said, I've come to prefer fixed parallax scopes on my rifles, it's just one less thing to mess with. I have 2 of the CP 3-9x32's with the 35 yard parallax setting, works really well from 10 yards out, although I think I may need more magnification. I also have a recently purchased Bushnell Elite 10x40, fixed power, fixed parallax. I readjusted the parallax on that to 50 yards, but I think I'm going to drop it back to 35 yards. Because it's a fixed 10X, it's touchier about short range focus, and I can't turn the power down to compensate. But I think a 35 yard parallax will give me what I want. BTW, your Gamo is actually a BSA, and they've been making air gun tough scopes for years. It may well hold up just fine on your 28, where a "better" scope would bite the dust. It's worth noting that I killed several Leapers scopes on non-magnum springers, one of the reasons I don't do Leapers scopes any more. Keep us posted.
Dave