The GM Cannon II utilizes the Gundam ALEX's Chobam Armor technology, giving it surperior protection in combat. Because it would've been too heavy and costly, engineers decided to use the Chobam as the primary armor of the suit. Unfortunately it cannot shed it in combat like the ALEX, but this gives the GM Cannon II less weight to haul and gives it exceptional maneuverability for its size. Its one of my favorite grunt suits of all time.
It is sad to me that the GunCannon and its variants have been discontinued by the Federation. I remember even seeing official art featuring Jegan Cannons that were never used. I confess that I always had a lot of affection for the more robust version of the Gundam, which didn't always have the means to show its values. In Unicorn, I remember seeing the GunCannon DT only being taken down by 3 Zeeks, as it was so resistant to their attacks.