You've had excellent advice from vgorst IMHO.
There are very few lizards that are actively social I'm afraid - most prefer to be kept on their own. A single gecko will not get "lonely".
Have you considered a Rankins dragon? They are very handlable and meant to be fairly tolerant of sharing and you would just about get 2 in that size viv. But only really feasible IF you get 2 females and IF they are from the same source (so that you don't have to quarantine them) and IF they are the same size. Even then you need to be prepared to split them if there are any signs of dominance or aggression.
Eublepharis macularius 7.18
Timon lepidus 2.1
Lygodactylus williamsi 4.0
Teratoscincus roborowski 1.0
Phelsuma grandis 2.2
Bellatorias frerei 1.2
Pogona vitticeps 1.0
Last edited by jools; 13-02-2013 at 01:36 AM..