Before the Great War, they could be found in warm climates throughout the world, range from 1.6 cm to 60 cm, with the particularity among other lizards to making chirping sounds in social interactions with other. After the Great War, they're mutated, causing them to grow giant and taking a humanoid form, because of their exposure to the mutated FEV of the West Tek Research Facility coming from the air, and some with other toxic conditions.
Little geckos have horrible detection AI, and it is often possible for the player to walk right in front of them and not be noticed. This makes it easy to get inside attack range without being caught. While it has been suggested that little geckos attack only when provoked, this is not true.
The golden gecko is the more beautiful, but also more deadly kind of gecko. As stated by Smiley, the Trapper from the Quest Rescue Smiley the Trapper, they can be found near toxic waste, which they seem to lick up. Though it's not said explicitly, obviously there seems to be a connection between the toxic/radioactive waste and the golden geckos.
The fire gecko is the most deadly kind of gecko in Fallout 2. They appear mostly in late game and, as indicated by their name, can breathe fire and are quite hard to kill and as opposed to both other forms can not be skinned, as their hides are far too mangy and burnt to be worth anything. Not surprisingly, they are immune to fire damage.