depends on the gar but I would say something like 10' x 4' x 4' for Florida gar and spotted. tropical, Cubans and bowfins would do ok in 8' x 3' x 3'. but alligator gar and longnose gar shouldn't be in an aquarium ponds of at least 3,000 gallons round is best for these species and they shouldn't be able to see through the walls of the tank.
I have kept 5 Florida gars in a 6*2.5*2,
for 10 years, grew them from 3" to 18-19", yes a bigger tank is always better, but a 220gallon will suffice, as these fish would get to 18-19" and pretty much stop to grow.