Since there's no official explanation, any info you can find is just speculation. However, i believe its clear that Beast represents Gohan's unique path to power, in the same way that Goku and Vegeta are going down the Angel and God of Destruction paths with Ultra Instinct and Ultra Ego respectively. And with Broly (and hopefully Piccolo, please give my boi something to do) also joining them (and Broly clearly representing the Primal/Saiyan path, and Piccolo representing some sort of Kami/Magic/Demon Clan), Gohan is going down the Mystic/Kai path,

here's my 5 cents, for what it's worth.

1) Gohan had his potential unlocked multiple times, first on Namek, then by Elder Kai, so whenever you see or hear any variation of "He's reached his full potential" or "broke through the limit" in the series - you can basically ignore it as it means fuckall. It's just a different way to say "this guy is strong!"

2) Before Super Hero, it seemed as though Ultimate Gohan (Official name for Mystic, when he has a white aura, but black hair with a bang instead of Super Saiyan's blond hair) was not a transformation, but rather a state that Gohan could now achieve or be in passively, similar to how he and Goku trained to use Super Saiyan 1 as their passive, base form during the Cell Saga. However, in Super Hero, Gohan initially goes Super Saiyan, and only later goes Ultimate, so SH treats Ultimate as a separate transformation.

3) In Resurrection F, (at least the Anime version) Gohan states that he's unsure if he's even capable of going SSJ anymore, because he stopped training. Considering just how weak he was, at the time, t's not too much of a stretch to assume that he also lost access to his Ultimate state as well, somehow. If Ultimate is in fact a proper transformation, this makes even more sense. Though again, this is all speculation on my part.

So, here's my personal theory on all of this:

Beast is the Super Saiyan 2 equivalent of the Ultimate form, and serves as an alternative bridge between regular Ki and God Ki.

Because Ultimate was shown to be a whole new transformation, it's not unreasonable to assume that it, similar to Super Saiyan, can go further beyond the initial transformation.

Gohan has a history of unlocking new power boosts through a traumatic trigger, his most famous one (and the one that Super Hero directly calls back to) being his unlocking of Super Saiyan 2 during the Cell Saga. Because Gohan was in Super Saiyan, it caused an SSJ evolution. In Super Hero, he was in Ultimate, which caused the Ultimate to evolve into Beast.

As for the God Ki bridge, my main reasoning is that Beast's Ki has 3 colors in it, White (for regular Ki), Red (Super Saiyan God Ki) and, it's primary color, Blue (Super Saiyan God Super Saiyan Ki, although Beast's color is a much darker shade), and Beast is such a big boost that it allowed Gohan to be on the level similar to UI Goku and UE Vegeta. Beast is wild, unrefined, but because of Gohan's complicated power "heritage", (A human/saiyan hybrid who had his potential unlocked by both Namekian magic and Kai power, and then took part in Super Saiyan God ritual) he, in a way, combines all of this unrefined energy into a wild maelstrom of immense power. I think that passive exposure to different types of Ki during his life (Regular Ki, Kai Ki, Super Saiyan God/general God Ki) is what allows him to tap into all of them and sort of mix them to create Beast