He's a tricky one to say.He's blowing hot and cold and his attitude can be quite effy sometimes but that's not saying he has a poor attitude as it's normal for teenagers.He's a different kind of player to Roberts, both of them are amazing dribblers but Roberts is more of a goalscorer while Gribbin is more of a creator/link up player although he's capable of scoring some amazing solo goals from times to times plus his free-kicks are usually very good.Both of them have very high ceiling (talented enough to potentially become regulars for their current club) but both might turn out to be nothing player if their development are not handled properly (Roberts is quite weak while Gribbin is quite inconsistent even for a player of his mould).
Him,Rashford,Williams,Tuanzebe and Fosu-Mensah are certs to become Januzaj/Pereira/Wilson of the next batch (i.e those who will be given proper first team chances not because of injuries).How good they will become are, as always, depends on a lot of factors.