When do you foresee the spring will "wear out?"
Is this a common event with Gamo spring-piston air guns?
For a $150 airgun, if you think its going to wear out a major part within the period of time it is likely that the factory will actually service it, and you REALLY like that rifle, you might as well buy a second rifle, and just stash it for the future. Toss the old one when it dies. These are not collectors items. By the time you wear out a piston spring, the spring is probably not the only thing on the way out in a gun of that price range.
Air gun service these days is not what it used to be.