Gale probably doesn't leave you because, unlike Wyll, he will literally blow up if you're not there to feed him magical artifacts.
When my evil warlock convinced Gale to stay, which happens at the goblin party, Gale has not yet confessed or asked for any magical items. At this point he doesn't even know if the main character will agree to provide any artifacts for him. And an evil character ruthless enough to destroy the grove and kill the refugees is unlikely to care about his plight.
