In my opionion it works like this. Endusers go to GD, GD says domain taken but we can try to get it for you, meanwhile they can get a price from the buyer also. And then they make price request at AN "on behalf of the buyer". Buyer doesn't know anything about that price request. You can add prices. Then the potential buyer (and everyone searching for the domain) will see the BIn and min (this part is great, but..), but not the floor (of course they shouldn't); but GD can block this possibility, and if you ask them why are you doing this, they will just ignore. Not a fair business. Your response to price request will go the buyer, but buyer here is GD and can ignore your response, and not transfer it to the actual buyer, thinking "not worth my time".
Or instead they can make offer at Dan or Sedo as well, who knows..
So two separate sales are taking place, you sell to GD, GD sells to the buyer. Ideally price for selling to GD should be the same as price for buying from GD, and theoretically you can check this after the sale is completed by asking the buyer; but there can be "exceptions" I suspect.
And of course others can also make offers, or make requests but probably they would also be domainers or agents, or endusers familiar with domain business.
Does GD ever advertise AN anywhere? No, because it is basically an interface between mid-people and sellers.
I don't understand why so many endusers prefer GD over competition.