I think the cals in gum are just the cals you get from chewing it. I don't think there's many downsides to gum, in moderation! The sugar-free type is probably best, good for saliva production and combating plaque after a meal. But I've heard that excessive consumption of sugar-free gum can cause some nasty side effects because the sweetener sorbitol can act as a laxative. Apparently people have lost a lot of weight and had some serious bowel problems after chewing too much of the sugar-free type.
I've also heard about people who over-chew having problems with their jaw - temporomandibular joint and all that jazz, so maybe be careful you're not chewing too much.
Maybe keep a little box of crunchy snacks in your desk drawer? A broken up couple of Alpen light cereal bars, fruity ryvita, brazil nuts, dried banana... even some fresh chopped up veggies can be good for satisfying the chew urge! A couple of Werther's original sweets interspersed with the gum might also help - they last for ages, not too high in cals, and it might help cut down on too much gum.