Although this is going to make this pretentious sounding, but just because public understanding agrees with something doesn't really make it right.
GL isn't really an epic, mostly because of what Cirno said--it has Simon overcoming a big enemy, sure, and saving the day--but that's not really much different than any other show that has a Hero's quest in it somewhere.
compared to an Epic like the Illiad, the Odyssey, Beowulf, etc., in the form of something like Yamato, you get some of the same things that you get in hero quests--but Yamato and the ilk carry social morals within themselves, meaning that yes you're following heroes here and that the heroes vision and the author/creator's vision is one and the same; this is "how" people should act or lessons they should "learn" by reading/watching.
GL is certainly a heroic show, but it's not really an epic in that sense. It's just a hero doing heroic things.