The main issue with Grandmaster being a Celestial is that he isn’t an imposing figure like the few Celestials we’ve seen so far.
Even though Ego was primarily represented by a humanoid avatar in Guardians of the Galaxy Vol. 2, his true form was an entire planet.
It’s also possible that like Peter Quill, Grandmaster is only partially Celestial, which would still make him a formidable threat to anyone who disturbs his games.
Jeff Goldblum’s Grandmaster may be powerful (one would imagine he has to be in order to effectively control a planet full of mighty warriors), but the Thor: Ragnarok previews haven’t indicated that his true form is something gigantic.