They are both CMs so its comparable.
And yeah, high risk-high reward. So it goes
There are different roles to CMs
Casemiro for example cant be compared to KDB
I mean if you want you can say KDB does a better job at his role than Casemiro does in his but when you create a team you dont just put 3 KDBs in midfield (Also because there is one KDB) but also because there is no one to cover for him.
Locatelli and Gravenberch? One is more tactically sounded and is really established in his role while the later is a talented player for his role but far from establishing himself.
If you wish to compare by roles then obviously Locatelli > Gravenberch at the moment.
Potential? Gravenberch seems like an explosive player so if he manages to mature in a way that helps him show his explosiveness more often, yeah he has a higher ceiling.
If we need to pick one for now - Locatelli will instantly walk into our starting 11 while Gravenberch would be the talented kid that comes off the bench and deliver some nice moments.
I must admit though Gravenberch has something I cant really point on but he reminds me a bit of Pogba.