well, to be honest, I don't have many headcanons regarding the two of them since I didn't pay much attention to them until they started asking me about them
but i have some! It's not a big deal, but it's something
1-gatherer and the pirate troll have a meeting point on a dock or lake (or well, it was an area where gaterer used to hang out until the cobalt troll invaded his space)
when the cobalt troll is bored or frustrated by something he goes to that area to be with gatherer, gatherer is more for listening while cobalt troll is for blabbering
2-when gatherer developed teeth, he liked to bite recurrently, currently he doesn't do it so often, but sometimes he does it when he is anxious or bored, he bites pencils, nails and even his shirt
3-this may sound rare but I like to think that the possible lusus of the pirate troll is a monkey with extra limbs
4-gatherer is not one to show much affection in public in part he feels uncomfortable, although the pirate troll uses that to simply annoy him