Depends on which gar, the Florida is the smallest. Longnose and alligators get massive, and are unsuitable for even the largest pond. If you want a Florida, you need a huge tank.
actually if you are looking for a small gar go tropical or get a Hujetta "gar" of course you will probally not find a tropical for sale.... I would go with the florida though they are one of the best looking IMO
hujeta gars....aka rocket gars.......are great top feeders. I've had one in my 125 african tank for 1.5 yrs and he is doing amazing.....grew from 3'' to his present 7'' +/-, but if you are adding this fish to an african / agrro type setup do so at the beginning when you first set up your tank....I have lost 2 hujetas ( not all at once ) that i have attempted to add to my african tank, but my original is still going strong and I feed him like a pig....pellets, live , krill, shrimp......anything........a good time....always hungry, can you tell i like my hujeta!!
ali gars get 10' plus so unless you are willing to devote an olympic size pool for this fish disreguard.........