GT may not be dbz's canon, but it is a spinoff so it does have its own continuity within the db universe.
Since Broly was part of dbz's movie canon which is a spinoff of the dbz manga/anime canon aswell as GT. These sentences I'm about to present to you are logically correct.
Broly was defeated in the Z movie timeline by a power of less than SSj 3.
If in the Gt timeline which is a spinoff of Z timeline states ssj4 is strongest saiyan, logic would dictate that broly cannot defeat or archieve higher than this.
Gogeta ssj4 is goku+vegeta once again very unlikely broly would win, broly got defeated by something less than even a regular ssj gogeta.
If this doesn't make sense to you, you're either A a retard or B a fanboy.