Either use a rope lure. Literally a piece of frayed nylon rope about 4 inches long tied directly to the line. No hook. Try to sight fish to them when they are sunning themselves.Don't set it, as it is only tangled in their teeth. Or you can use a rod with a lot of backbone, a medium sized 4-8 very sharp hook, a small float about 12 inches above the hook, a bit of splitshot and a live minnow. Cast to the sunning gar and they will take it almost immediately. Let then run with it for a good bit to ensure that they get it down into their mouth a bit, and not just in their bill. Then set the hook like you are trying to pierce armor. Often you will just pull it out of their mouth, but a percentage of the time you will get a good hookset. Pinching the barb down or filing it off is essential to removing the hook with any ease at all.
Shortnose are much easier. Not much more difficult to get a hook set than a bass.
Good luck, they are fun when you catch them. Some fight pretty hard.