Gecko Skinning is a Fallout 2 perk.
This is a special Perk, so it cannot be taken at new levels like other Perks. It can be learned only from Smiley the trapper after you rescue him from Toxic Caves.
You know how to properly skin geckos (except for fire geckos) for gecko pelts and golden gecko pelts. You can then earn a bit from selling their skins.