Most devil’s have the feature Devil’s Sight:
Devil’s Sight. Magical darkness doesn’t impede the [devil’s] darkvision.
The Gloom Stalker’s Umbral Sight says:
…You are also adept at evading creatures that rely on darkvision. While in darkness, you are invisible to any creature that relies on darkvision to see you in that darkness.
By my interpretation the Gloom Stalker will remain hidden even from a devil when in darkness, however my DM believes the devil will be able to see the Gloom Stalker with just their darkvision, as the effect is magical.