Comparisons need to be like for like. Alli is a 1st team regular,probably the 1st name on the teamsheet along with Kane. He's been young player of the year (twice) & is a full England international.
Gray is a substitute.
Comparing him to someone else is only difficult because the other player would be someone we'd likely never heard of or know very little about.
I like Gray personally but he's just a lad with potential which will be unfulfilled unless he adds some end product to his game. Light years away from Alli currently. As for the future, who knows? Hopefully he'll be a superstar but no way to tell as of now.