I'll pass along what I've learned over the last several years. First, NEVER introduce ANY kind of oil into the compression tube through the transfer port. Just in case you're not familiar with air gun terminology, the transfer port is the small hole behind the pellet that the air goes through to launch the pellet. The reason for this is something called detonation. The pressure spike created by a springer creates enough heat to ignite any oil in there. This is bad for several reasons, at best, it screws up accuracy, at worst, it can damage your rifle. About the only use for oil in a springer is at the pivot bolt on a break barrel, and then you use just a tiny amount.
To effectively and properly lubricate a springer really requires taking it apart. If you bought your Gamo in November, taking it apart would void your warranty. Or do they have a "lifetime" warranty? Most of us don't care, because the improvement in almost everything from a basic lube/tune is worth it
. But you do need to be at least moderately mechanically inclined, and there are some additional tools and materials required. Do a search on tuning for more info than you would ever want
If you're not comfortable taking your rifle apart, you CAN do what's call a slot lube. That consists of pulling the action out of the stock, and putting some moly grease or heavy tar on the spring through the cocking slot. Don't get carried away here, this is definitely a case where less is more. A VERY thin coating is all that's needed or wanted. It's not something I consider very effective, but others have said it can help, so there you go. Moly and tar are available here:http://www.airguns.citymax.com/page/page/251327.htm
Look under lubricants.
The other option is to have it tuned by a pro, but that will probably cost as much or more than the rifle did. Up to you whether it's worth it or not, but most think it is, even on less expensive rifles like a Gamo. BTW, even high end springers frequently need a tune-up, and almost any of then at least benefit from one. Hope that was some help, later.