Nice looking 'G' and an interesting display. As this is a 1935 date, and you are in Spain, what is the chance it was used by German troops in Spain's civil war??? Was it kept inside Spain??? Do you know its history???
My method of loosening stuck screws is to clamp the stripped gripframe to a milling machine table and use a screwdriver bit in a Jacobs chuck in mill quill to manually turn the screw out. Soak screw in lubricant (or lightly from back in case of grips), lower quill w/bit to screw, set quill lock, and turn screw counter-clockwise by hand to free screw. You may have to 'rock' it back & forth.
It usually works...
