Do Brands count as your damage or is it like totems, traps, and mines? Will there be a Brand Support gem just like how there’s Trap Support, Minefield Support, and Spell Totem Support?
The Brand is a spell you're casting that directly deals damage to enemies. It's your damage, just like if you cast Orb of Storms or Fireball and they hit enemies. It just has a slightly more complicated way of working out which things you're damaging.
Of course, if a totem, trap, mine or minion casts the Brand, then they're the one dealing the damage.
There is not a currently support gem that makes supported skills interact with brands, and cannot be one that works the same way as the Totem, Trap and Mine supports, because Brands are fundamentally very different - they are themselves a spell cast by you that's damaging things repeatedly, where Totems, Traps and Mines are separate enties that themselves cast spells, and have those spells deal damage.