>>23580991 (OP)
Make it a tank-type mobile armor, instead of a glorified pseudo-MS with larger armaments.
1. At the minimum double its size, and plate it with capital ship tier armor, so that it's at least as durable as a MS-sized tank. Then when you put it up against overwhelming firepower, the Gundam and Guncannon can use it as mobile cover. Hell, even add a vehicle compartment to the back so that it can carry troop transports, assault teams, and the like.
2. Mount multiple point-defense vulcans all over, so that it can shrug off missile attacks with ease. I would also say I-Field generator to protect it against beam attacks, but that may be going over the top, especially since Zeon didn't actually adopt beam weapons until well late into the war.
3. Upgrade the back cannons to capital ship style beam cannons with the same firepower as a Salamis' beam turrets. This way when the enemy brings ships like Gaws and Gallops into the fight, the Guntank can shoot them down.
If it's going to be called a Guntank, then sure as hell it'd better be as powerful as a tank upscaled to MS proportions.