These robots vs Galactus thing are getting pretty funny, they can't defeat a well written Galactus, he'd probably add them to his tech somehow, or make Brainac realize that he would be better off serving Galactus than fighting against him.
Even if they wouldn't join him Galactus is an essential part of the universe, and they are far out of their league here, he'd drain them both as easily as he did to Ultron.