Gareth would definitely like Wendy and while his knowledge of her type of magic is pretty much nonexistent, he would be happy to share what he does know. He'd still probably be a little cautious of her due to her immortality but her good heart would eventually win him over. I can see Gareth being very amused by Peter and Wendy's little family.
Gabriel...Gareth does not like Gabriel. However, Gareth likes to disguise himself and sneak aboard the Black Revenge every once in a while so when he is pirate!Gareth he'd probably be much more agreeable.