Gomez,
Just had to deal with a leaky gas tank myself. If the hole is small, you might be able to use a product called "Seal-All", which is my favorite. The stuff looks like airplane cement. You put a dab on your finger and press it on the leak. Allow it to set for awhile and apply a second layer. I usually use several layers and finish off with a small piece of milk jug or other thin plastic stuck on with a generous dose of Seal All, cover that with a small piece of duct tape. The stuff works good at stopping leaks, the plastic and tape is just to keep it from being rubbed off. This stuff has stopped leaks for me even when fuel was squirting out the hole. Actually, that's the best time since it's easy to find the leak and make sure its stopped.
For larger holes, you really need to use the epoxies, either a putty or something like JB Weld. Places like JC Whitney also sell a two part tank liner that you mix up, dump in the (dry) tank and swish around. It's the home version of the commercial gas tank repair. Don't know it it works, never have had the opportunity to try it since the seal all works so well.
SHF