A Gary-Stu is often defined by being a lot of things, but we’ll focus on the stuff that Flash exhibits. He has no noteworthy flaws (beyond being ‘adorably clumsy’) that make attaining goals or interacting with others challenging, and his only real defining character trait is that he is Twilight’s love interest in the movies.

A character’s flaws are promises to the reader about the future story development. If a character has problems standing up for himself, the issue will likely be addressed later on. He will get confronted with his problems and either overcome them or work for a way around it. If you have a Sad or Dark story, he might also crumble under the pressure. No matter what, it’s character development and therefore part of an interesting storyline.

If the character keeps overcoming his flaws, he will eventually get flawless, but it won’t make him a Mary Sue.

Contrary to that a Sue is a character that gets implemented in a way, that makes such growing impossible. If he has no fears, no physical or mental problems, this is a guarantee to the reader that the character cannot develop and will stay flat for the entire story.

By this definition Flash is a Sue. He got introduced as a mix of different positive stereotypes. He is popular, has enough money to buy an expensive car, attractive, has a nice hobby and yet despite all of that he’s still humble and nice to Twilight even though he doesn’t know her and she seem kind of weird.