Ok so I changed it a bit and I made him keep most of his personality traits abilitys only making him more violent and hungry as well as want the same things the void wantsIf fate is effectively death be it physical or mental or personality or anything like then it has to overcome his immortality.
Effectively if one can look at Gobbus and say that this is Grobbus as he was conceptualized and created then it would not trigger his immortality. He can be changed but so long as the core characteristics of what was Gobbus.
And the Pit can understand patience, it just so happens to be soo hungry that thinking like that for long is difficult and can see the benefits of having someone act as to direct it while it wallows in his hunger. Gobbus also has the dual benefits of being immune to the Void's touch. So the Void actually having influence over him would be a breach. Remember that I summarized the words of the bargain but his first wish was total knowledge of the workings of reality and the events going on in the universe. So he formulated his immunity bargain in such a way that none can directly affect him.
So the only way the Void can kill Gobbus is if the Three no longer hold supremacy over the Universe.
Yes. Gobbus the demon King and Gobbus the slave are still the same people.