I have an older GT Avalance that I bought without a front susupension. I am looking to add one to it, but the guy at my local bike shop said it would throw the bike way off by putting the tire out too far forward. Said the bike was not "designed" to have a front suspension. Is he full of crap, or am I really not going to like the outcome of adding a front suspension. I love everything else about the bike. Deore everything etc etc.
What year? He's referring to the head tube geometry, which affects the handling characteristics. Some bikes built in the suspension fork "transition years" were built with both rigid & suspension forks in mind.
If you can find a 60mm travel fork you're likely not going to throw the feel of the bike off much. Put a 140mm travel fork, and you'll feel like you're on a chopper.
Watch out! I don't know GT's history, but your frame may require a fork with 1" steerer tube, which are getting harder (i.e. more expensive) to come by.