A 90 gallon is going to be too small for an adult of any species of gar, but it will work for some time for a Florida gar. Florida gars are the most commonly available gar in the hobby and are a good first gar. They also the gar that is most often advertised as another species of gar, such as a spotted gar or alligator gar, for either monetary reasons or just an incorrect identification. The majority of gars sold as spotted gars are actually Florida gars; they tend to be IDed as spotted gars on account of their pattern and a general lack of easily identifiable external differences from the spotted gar.
So in short, don't get a true alligator gar; do some more research and consider getting a Florida gar.