I wouldn't recommend any true gar for a 125 its not wide enough at 18 inches wide
the ones for small tanks I recommend (if you can find them) are tropical gar (Atractosteus tropics) and Cuban gar (Atractosteus tristoechus). they both stay small in the 2-3 foot range. but gar are not good aquarium fish. if you want true gar start with a 300 plus gallon that is at least a few feet wide. stock tanks work well
A Florida can live for years in the 18" width. 2ft wide would be better but 18" can last for a while. Florida gar is ideal, spotted and shortnose, nothing from the atractocus side
I have a striped pike characin and a rocket gar in my 125 gallon. The striped pike is at around 12" and the rocket is approaching 9". I am getting out of the hobby, not by choice, I need to move and my new place doesn't have room for the tank. I am waiting for an offer currently but may be posting them on the marketplace soon!