So it's not about his development as a player then? It's more that you think the team would be better with him at 6?
I'd come back at you and say that Gray is the second best option we have fit at CB right now. If we had a fit CB, I suspect we would have seen him play 6 against Southampton. He has had some minutes at the #6 this season but he's still 18 and my guess is that 99.9% of coaches would be relying on their senior pros like Bissouma and Bentancur before throwing Gray straight into the firing line. It's his first season in the Prem, if you throw him in instead of bedding him in gradually, that's negligent management IMO.
I get that we saw him boss a very dysfunctional Chelsea team in league cup game and that absolutely shows the potential he has as a #6 but it doesn't mean you throw him into 90mins week in week out of playing CM in the Prem. I have zero doubts he ends up a #6 for us and also for England IMO but these appearances learning FB and CB are only going to help him understand the positioning needed for that role even better. If he's 22 and we still don't know where he's going to play then it's time to look at it IMO. Right just get minutes and learn the game at the PL level.