This fight really comes down to which you believe is stronger. Neither have feats to suggest they could kill each other. Although, GEB does have feats.
One thing that suggest that PM is very very strong is that he views the universes as germs, but they have something very powerful which is stories. The fact that he views them as germs should indicate that they are nothing to him.
However, if GEB is just like the Presence, then there is a big problem. The presence told Spectre that everything is a toy to him. Nothing is equal to him.
GEB... I remember a scan somewhere when Spectre (idk if he was full powered or what but he got wtf pawned). Spectre tried to stop GEB from entering his dimension or something. Spectre was trying his best to push back something that was a size of a mountain and wasn't doing jack to it. They later said that the massive mountain spectre couldn't push back was a tip of a finger of the GEB.
IMO the GEB is atleast equal to the Presence or on some greater level.
Spectre or the presence says the GEB is the total absence of good and light... like idk its the adsoulte supreme being of "bad/evil." Idk if they used the term "multiversal" so I can't really say.
Maybe the GEB is beyond the scope of the multiverse or within PM himself, no clue but GEB is incredibly powerful.
I haven't read much about Primal Monitor so i can't relaly judge this fight. But would love to know who wins.
Then by the same token you would have to ban Thanos w/ HOTI since he would shit all over Mandrakk and CA Supes. While your at it you'd have to ban TOAA, PR Beyonder etc. They aren't too powerful as I just named one character who craps all over them primal Monitor excluded.