i would go with most of maddings suggestions, except if you would like to use live plants floating water sprite, hornwort, or the like will work just fine. also java moss or java fern will work well provided it is anchored (i have used all these plants with gars in the past and present and they work well...particularly the floating water sprite - this works well to shield the gars from light and also absorb nutrient waste from the water).
width of tank is important as is a good filtration system, although if you can keep up with 50% water changes ever 1-2 weeks that will help with the system provided the overall volume of water is appropriate for your gars.
this is also relative to the species you are keeping...gars in general are hardy, but some are more durable than others...Florida and tropical gars are probably hardiest, with spotted/longnose/alligator second in that group...keep in mind gators are most aggressive and will need larger tanks. shortnose gars are most skittish and very prone to breaking their backs. Cuban gars are probably most sensitive to water parameters, particularly pH, so keeping them in higher pH or well-buffered systems is key.
keep driftwood and other tank ornaments to a minimum...allow for as much open space as you can. with decor to a minimum, a lot of the rest of the housing depends on the species you want to keep--
--solomon
