We used two different pistons hooked in series for our hatch grabber. The first piston was a 3/4" bore which produces around 25 lbs of force @ 60 psi. The second piston was a 5/16" bore which produces 5 lbs @ 60 psi. This made for a very soft landing when we went in to place the hatch.
We had both of them extend and retract at the same time (on the same solenoid channel) but we had talked about moving them to different channels so that we could fire the small piston after we had made our approach to “plant” the hatch rather than using the driving forward speed to plant it, but we never did that.
Anyway, for this problem, if you removed the gearbox from this arm and had a second arm driven by the same gearbox located behind the first arm (well inside the frame perimeter) and then used a piston to connect the first arm to the second arm to create a parallelogram linkage between the first arm, then the impact forces would be absorbed by the piston and would not backdrive the gearbox. You would not technically even need an active pneumatic system. You could just connect one side of the piston to a tank and then pump it up to some moderate pressure and then seal it off. This arrangement would allow the piston to absorb the impact and then push back out to full extension afterward (the tank would absorb the working air and then return it to the piston).