There is a chance that it's always been his true story. He could've lied to our MC in his revelation, because the details of obtaining "the netherese book" are vague. There is even a question I think, something a long the lines of "And the wizard happened to have this book on hand?" - to which Gale replies by brushing it off with some general comment like "Hoho haha, thats Waterdeep for you mate, the city if magic". Doesn't feel very believable, does it? Thus, I don't expect this story to change much. I think man is a liar and a sucker for secrets
