I have nearly 16,000 rounds through my .25 caliber, WAR Cobra. It has NEVER been cleaned, and shoots better than 3/4 inch group at 100 yards at the indoor range. BUt...
Some barrels just get dirty for a variety of reasons. I never lubricate my pellets, but some folks think it is the cats meow. If you do, you have to think about dieseling, no matter the driving force.
The Gamo is a break barrel. I personally have not had good luck with ANY nitro-piston airgun. If you are, good luck. However, they tend to shoot best with the heavier pellets, but be warned. What shoots best in your friend's identical airgun, might not shoot well in yours. After all, hold is as important than any other attribute you can name.