In the art piece in which I first introduced these new outfits for these two Griffons, I mentioned my headcanon of Gallus going all "tough teenager" at some point and Grandpa Gruff trying to be reasonable with him and warning Gallus that he won't find what he's looking for on the streets, because he was once there. The story takes inspiration from the episode "Rebel Without A Glove" from The Looney Tunes Show, but there are a few differences.

The way I see it, Gallus wants to show other Griffons that he's not a meek punching bag and that he's a strong Griffon, which is why he temporarily changes his name to "Gremlin" after finding out "Gallus" means "rooster", but later finds out it also means "mischievous", while Grandpa Gruff wants to show other Griffons that he's indeed calm and collected, and not some old crotchety guy named after his temper/ insanity, which is why he tends to go by his full name, which is "Gruffudd" — for anyone wondering why he's called "professor" here, you'll find out soon enough. ;) So, without even noticing, the two practically switch reputations and personalities for a little while. Even when they have changed, they still call each other their usual nicknames ("Gramps" and "Boy") and find some ways to get along and deal with each other. Followed by this, even after the two Griffons switch back to their previous personalities, they still occasionally wear those clothes, both because they like them and because they help them express parts of themselves in another way. Gallus feels able to show his inner confidence and toughness, and Grandpa Gruff feels able to show his more forgiving and serene side.

I'm gonna continue calling them their normal names in future drawings of them with these appearances so there's no confusion.