GUs are formed when multiple universes in a multiverse or megaverse combine. Since a GU is the size of Graham's Number, you would need a Graham's Number of universes to form 1. This is only possible in a megaverse and not a multiverse. The ages of each universe is amalgamated together; if 2 universes that are 500 years old are joined, the final universe would be 1000 years old. How the universes combine is similar to a black hole joining with another; two universes will pull each other into each others' gravity until it turns into one, much bigger universe. This will send gravitational ripples strong enough to rip apart multiple universes across The Box, and also why GUs are usually rare.
A GU reaching the age of Graham's Number will immediately collapse, similar to a supernova in 2 attoseconds. The explosion will be unbelievably violent; more violent than a Big Rip and Big Crunch. Two less violent explosions will appear where the center of the GU was located and leave a disc residue. A universe can form here, although unlikely.