I did not remove the piston seal (parachute seal) but it looks like you would have to put the piston rod in some soft jaws of a vice and undo the screw holding the seal on with a large screwdriver. Clean everything up and put it back together. Put piston and mainspring in the cylinder and then the guide and its block (on this one the guide came out of the block but maybe some are in one piece?). Push down on the guide and then locate and screw in the stock screw lug. Refit the arrester and screw on the scope rails. Locate trigger unit and end cap and refit the two pins. Slide breech block/barrel and shims into the jaws and locate the cocking arm at the same time. Replace barrel pivot bolt. Drop action into stock and pull safety arm down and wangle the trigger guard into place and replace the screw. Replace the two front stock screws.
I dont really like gamo,s as I find then pretty souless AND some of the bolts,screws and pins can be really tight. This came with a spring that seemed too small in diameter for the size of the piston so I fitted a larger diameter one of a similar length. As the sights were missing I tried an ASI 4 x 20 with a single post. After 6 or 7 shots that dieseled , the gun settled in and seemed pretty good for plinking. I reckon a better scope would help (even an smk 4 x 20) but this was a period piece. Not my cup of tea but ok for nostalgia for someone. Trigger unit looks like it would be easy enough to strip but I have not checked whether you can get hold of the parts.