Before explaining the Z'Gok, let me explain a misunderstanding I had.
Including Mobile Suit Gundam, robot anime of that era were driven by toy sales.
Therefore, it was common for the protagonist's robot to transform, power up or be replaced midway through the story, and for the enemies to change each time.
This was fine before Gundam, but Gundam has aspects of a fictional war story, and the appearance of the mechs from previous series would be inconsistent.
The story begins with Episode 1, "Gundam Stands on Earth," on September 18, 0079, and the enemy mobile suits that appear are Zaku, Gouf, Dom, and other mobile suits that would appear later.

MSM-07 Z'Gok and the Secret Story of Beam Weapon Development | nobu(BN界の鳥肌実)'s Gunpla works