I think I remember Keith Law saying at one point that he somewhat arbitrarily set the cut off for "ace" as one of the top 15 pitchers in baseball. Not all teams have one, but if you assume the best teams have one (or more), that sounds about right. Gray isn't close to being in that group, but he could crack top 50 depending on what metric you're using.
Fwiw, Fangraphs projects him at 64 by WAR this season, with Joe Ryan coming out the highest at 33, but I'd bet against them on that one if he stays reasonably healthy.