Gars are not picky and can live in the most harshest of water conditions. I recommend using fine to medium natural gravel of about one inch in depth. When they are babies and really small they like to lay at the top of the tank in slow current next to something like plants or driftwood. When they get bigger they like to lay and rest at the bottom of the tank and periodically go up to the top of the tank to get some are and cruise around. Also make sure they have enough room to turn around. They are kind of stiff when it comes to mobility and if you have too many obsticals in your tank they might get banged up. Hope this helps.
If you want to put live plants get flourite (MAKE SURE YOU WASH IT FIRST, or it will leave dust residue all over your tank). But Gars aren't too picky about substrate