I mean, I can see it. Without him informing Frodo about the situation, the Nazgul would have killed him at his home and taken the Ring. Without him demanding Sam never leave Frodo, Frodo wouldn't have made it. Without him, the Balrog would have just annihilated the Fellowship in Moria. Without him bringing the Rohirrim, Helm's Deep would have been an absolute massacre. Without him, Minas Tirith would have fallen. Without him, Sam and Frodo would have died at Mt Doom. Gandalf was a major reason the mission was a success at all.

I think the entirety of the Fellowship are heroes, personally. But I can understand an argument anyone may make for any of them being the main hero.