if they havent been fed in a while it is very likely that they may take a swipe at your fingers. with small gars this is not really an issue as their teeth are unlikely to break the skin, but with medium-large gars (about 7" and up...also depends on species as gators and Cubans will do more damage than longnoses of the same size) they can easily draw blood with their teeth.
they won't amputate any fingers or anything, but they can definitely cut you up a bit. your best bet is to not mess with the tank when they havent been fed in a long time, or if you do, just put your whole hand in and once they see that it's bigger than just a finger, they will usually stay away. just start off working in the tank on the opposite end from the gar, and don't dangle your fingers in front of its face. i often work in my biggest tank which has gars ranging from 16-20+" and have never been bitten...they will eventually learn what is going on with you too. hope that helps!--
--solomon