Gabe liked his hair. It was a source of pride or even vanity for him, if he was honest. He could cut it... there were hair growth potions that would regrow it, although those tended not to produce as good effect as growing it properly - stronger that way. And its strength was one of the things he liked most about it.
The feather prank had gotten him fairly well, all things considered. A long shower had gotten most of the feathers off of his body - there were sometimes advantages to being largely hairless below the neck. He appreciated a nice happy trail himself, but his vanity wasn't so much that he would use potions to try to get one... he'd just enjoy them on others.
All of this meant that while he was able to remove most of the feathers from his body, but the glue used to affix the feathers didn't come out of his hair nearly so well. If Lyra wanted to help him remove it... he wasn't going to discourage her. He enjoyed people touching his hair - for any reason and he was smiling when he came upon the meeting place. "Hello beautiful."