Pretty much anything too big for it to eat, but you'll need a larger aquarium. Try to avoid shiny, shimmering, silver fish though, gars tend to attack them although I've seen them kept with large silver arowanas and payaras fine. Also stay away from plecos as they will suck the slime coat off of the gar.
How is this even possible? I've kept tons of plecos with my gars. What, the pleco is going to swim up and attach itself to the gar, and the gar is just going to stand there and hang out while its coat is scraped off? Seriously unlikely.
It sort of blows my mind that this could be a real issue, but people seem worried about it.