I don't think this GB will have people selling winch-capable (aftermarket) bumpers. Not everyone changes from one aftermarket bumper to another, like my dumb@$$.
It wouldn't be too uncommon for someone to upgrade to a full bumper from one of those hidden winch mounts (like the RockyMtnX one, or the new one made by PRG), but there aren't many people around our neck of the woods running those anyway so it's unlikely there would be one to buy as a result of this GB. I can only think of one So Cal Xterra Guy (Sage Bromax) running one, and I don't think he's planning to run a full replacement bumper.
Your best bet would be to order a new bumper with this GB and save the 15%, or just (very patiently) keep an eye on the for sale threads of various forums until something comes up. As you've likely already discovered, aftermarket bumpers don't go on the resale market very often.