No, fans don't think Gil is unbeatable. The VN shows him being defeated three times, and everyone I've seen has been arguing he can be defeated. What you can't do is make a character who is more powerful than him in general, who can beat him by sheer force. However, as you constantly argue whenever powerlevels are brought up, being more powerful does not mean automatic victory. Gil's ego gets in the way, and a clever character can use that to defeat him.