It's probably because you didn't let him join the group. I've met Gandrel before knowing Astarion's a vampire, and you can confront Astarion about it if he's recruited. If I remember right he says the monster hunter was lying. And if you go this route you get a dialogue option to say the monster hunter was right, on the night he tries to bite you. In patch 6 anyway, I haven't tested it with this patch.
