I can't find anything on what the Duke of Gloucester gets up to after dismissing Arthur's claim to kingship at the tourney in 510.

According to Greg's lands and leaders section he is no longer ruling Gloucester after 510. Did he throw in with Lot?

There also seems to be a 3 year period between Escan's death/ousting and Duke Morvid assuming control.

I'm thinking Gloucester rebelled, got killed in battle and his lands were seized by either Alain or Arthur (more likely) until a suitable candidate (possibly Escan's heir but possibly not) was found.

The extra cash from Gloucester no doubt helps in the early years of war.

In my GPC Goucester has been a real thorn in one players side. There was some unpleasantness over a woman which ended with Gloucester betraying the PK to Cerdic of Wessex's paid assassins (another old score) and the woman committing suicide to avoid marrying the creep.

The PKs son is understandably miffed (Hate 21) and already had to roll it opposed to loyalty (lord) to not attack Gloucester in London.

Meeting Gloucester on the field at Bedegraine could wrap this arc up nicely (well, until his son comes of age.....)

Looking at Gloucester on the map in the GPC it appears they were siding with Arthur.

Maybe he had to row in behind Alain or face a kicking?

Skullduggery at Bedegraine might be even tastier if the pair are nominally on the same side. Hmmmmmm