Activision Gets Weighty With Heavy Gear 2 is a Next-Generation Online piece discussing plans for Activision's sequel to Heavy Gear with the game's director, Jack Mamais. Thanks Apache. Here's the answer to one question that defines the distinction between a Gear and a Mech (as in MechWarrior III), which he feels defines the difference between the two games:

"Personally, I don't consider MechWarrior 3 to be a direct competitor. They are using mecha that have huge scale. They are capable of withstanding enormous amounts of damage and still remain walking. Heavy Gear 2, on the other hand, is much more fast-paced. Your gear is very agile and you'll have to use those abilities to keep you out of danger. I guess you could say piloting a mech is like driving a battleship, and a gear is like a PT boat."