My experience with Gamo is a sample of one, a compact target pistol
From my experience I wouldn't worry about the metal, I'd worry about the plastic. The plastic strap that keeps the barrel in position would loosen up over time, which was really more of an annoyance than anything. The true problem was the plastic grip frame cracking. Unfortunately they no longer make the gun so I'm SOL on a repair/replacement.
It's a wonderfully accurate gun but if you are looking for a long term air gun, I'd pony up for a German air gun. I have a 30+ y/o Beeman R7 that asside from replacing the seals is as good as new and I expact will last for several generations.
The German guns may be double plus the price, but IMHO they are ten times the gun in terms of quality.