Length of game time between rests does not determine the level of difficulty. The difficulty is determined by the number of resources expended between rests. More frequent and more difficult encounters are where difficulty comes from.

Gritty Realism just chews up more time in the game. The DM will give you time to rest, or they won't. Gritty Realism doesn't change that.

That said, you should be able to play whatever you want and it be no different than standard rests. However, if your DM plans to challenge you more by giving you more encounters daily, then I would lean towards classes that are effective even when their resources (save HP) are spent, like fighters and warlocks.