i agree with polypterus, you're best off not keeping the fish with anything, unless the tankmate is actually larger than the gator gar. even with that size tankmate, you could still readily have the gator attack it.
alligator gars are different in demeanor than any of the other species, so they definitely cannot be treated the same. keeping it with any fishes smaller than it can easily result in maiming of other fishes if not fatal wounds (or just plain old predation).
another thing to consider is that a 120g tank will very quickly be too small for an alligator gar. if you instead have a Florida or spotted gar, you will have a longer use of that tank and higher potential for tankmates--
--solomon