most gars would outgrow a 100gal, or at least get too large to live comfortably in a 100gal. but it would make for an excellent grow out tank. could you elaborate a little on the actual dimensions of the tank, the tankmates you plan on adding the gar to and any possibility of upgrading the tank?
if what you have is a standard 1.5' wide 100gal, then alligator, cuban, longnose and tropical gars are definitely out of the question.