One problem with the Gamo design is that they fall too far back when hit. When you pull the string to reset them, a lot of the force is resolved into a horizontal tension, and too little into a vertical motion.
Our club has a standard modification to the Gamo design. We drill a 6mm hole in the faceplate about 3cm below the string anchor point, and put in a 6mm hex head bolt approx 30mm long, with a nut either side of the faceplate, and the head of the bolt positioned so that it contacts the paddle stem when the target falls. You can adjust the projection of the bolt head by using the two nuts, so that the plate falls to about 45 degrees and no more.
The second modification is to strip out and throw away the two 4mm pins in the hinge mechanism, and replace them with M4 stainless steel bolts, stainless washers and stainless locknuts. Now you can adjust the locknuts to give just enough clearance to allow free movement, without it being slack.
The parts are not expensive and it takes maybe 15 to 20 minutes to do the conversion, if you are geared up for it.