Evening Hutchy,
Its pretty much like Hoss and Savell said. Big rivers and big lakes, but there are some that are known for it. I fish a lot in the Brazos watershed and for fun we'll try to catch some gar. I've never caught giants though. Most of them are 4-5 feet long, but still impressive. 70-90 pounders. Worth hanging the head on the fence post.
When I used to work for the state, we would do alligator spotlight counts on Choke canyon. Lots of times we would see really really big alligator gar while looking for alligators. Choke has some dinosaurs; particularly around the areas of Daughtrey WMA. Lots of giant dinosaur alligators too though. No way I would ever swim in that lake!
Interesting you brought this up, cause one of my buddies just sent this to me today. I can't verify it, but regardless its a Giant SOB. Says it came out of Toledo Bend.