As far as I know Gamo never made a bb gun named the FIX. They did make a model named the CFX, but it is a rifle and shoots pellets, it's not a bb gun.
I suggest you contact Gamo at the link below and ask them.
Gamo extreme hunter
Best left to a trained person.
You need to state the model and what is wrong with it.
Please state the model and the condition to get an estimate of value.
See the link below for the parts drawing of the Gamo PT80
Ahh figure out what the problem part is than figure out how to take the gun apart and put it back together from YouTube, an airsoft forum, or from an airsoft website. Than buy the replacement part and put it in the gun. If the gun was bought for under $100 than you would be better off buying a new gun instead.
If you disassembled it you may want to contact Gamo directly and see if they can help you with a diagram. Call 954-581-5822 Gamo Customer Service. Unlike Crosman who post schematics for all their guns and rifles, Gamo and Daisy does not. Your next best shot is to check the web for an "Air gun repair" shop and send it to them for assembly and repair.
Gun shop or gunsmith
Take the bb gun apart, memorize or take a pic of the inside, find the jam ,and put it back togethor
It might fit, but why would you want to do that. It has no gun powder and won't fire.
That will depend on the seller of the gun. If it was a new gun, they could no longer sell it as new, so do not expect a full refund. If there is a defect in the gun, some sellers will take it back, fix it, or refer you to the maker of the gun for warranty service.
Any air gun or rifle that uses a .177 Caliber pellet can use the Gamo .177 Red Fire Pellet. No matter who makes the air gun. The Caliber .177 is universal to all .177 caliber pellet guns or rifles.