Go to Lowes-Depot and feel the heft of a Full Caulking Tube of GE 100% Silicone Clear. Consider the amount of space you want to fill in the butt stock and Fore arm of your Gamo.
One to 1-1/2 tubes is about all you can get inside one. It will add considerable heft to your stock, which I assume is what you are desiring.
You can attach an extension of plastic tubing, (6 inches?) to the end of the caulk tube and reach into deep spaces otherwise inaccessible. Start at the deepest point and squeeze out silicone as you slowly withdraw the tube towards you to fill all the recesses.
I used non hardening window glazing compound in my Gamo S1K. But upon reflection, I believe this to be a much more efficient method.
Tube of 100% Silicone $5 each, Caulking Gun $2, 12 inches Plastic tubing (fuel hose) $1.
Now my Gamo feels and shoots more stable. A good investment. Go for it, you will be glad. (I performed many other Mods as well, guides, top hat, drop in trigger, tune, de burr, polish, etc.).