>>213103629test audiences saw this ending. it has been confirmed by multiple sources. disney didn't use it because the reaction of test audiences was overwhelmingly negative.
MCU movies normally film at least two different versions of critical scenes. They scenes screened for test audiences and Disney uses the scene that had the best reaction.
in the bad ending Galactus is overpowered and comic accurate. He fights the surfer and f4 with all his powers and easily beats them. They quickly regroup and the surfer convinces Reed and Sue to leave with Franklin because Franklin's power can be used to save other world from Galactus once he gets older.
Reed and Sue board the spaceship while the surfer, grimm, johnny hold back Galactus. Johnny gets killed and Grimm goes berserk. Galactus knocks Grimm into space. The surfer sacrifices herself to temporarily immoblize the galactus by hurling herself into his chest. This gives Reed and Sue enough time to escape on the ship.
After the poorly received screenings the ending was changed so that both Johnny and the Surfer are technically alive. The surfer lives on with Galactus at the far reaches of space.
Gardner is going to film some scenes for Secret Wars. it hasn't been leaked if she's the same version or an alternate version. regardless she's pretty much confirmed to be the MCU's surfer at this point. Rad will not be replacing her