Gum probably lubes your throat a little for the first few minutes, but after it loses it's flavor it probably dries your throat out! It's probably best to stay away from gum while you are on stage. Of course, PLEASE chew some gum when you get off stage to avoid what I call "Singer's Breath".
Singer's Breath is something I believe occurs after singing because you are pushing air up through your system, past your throat and into your mouth, and it's collecting all that moisture and particles from inside your body thus causing bad breath. Of course, this is just my own theory, but I am always paranoid about bad breath.
Also, chew sugar free gum because the other kind is bad for your teeth in the long run.