The Guiding Bolt says:
A flash of light streaks toward a creature of your choice within range. Make a ranged spell attack against the target. On a hit, the target takes 4d6 radiant damage, and the next attack roll made against this target before the end of your next turn has advantage, thanks to the mystical dim light glittering on the target until then.
Does this "dim light" on a Gloom Stalker ranger makes it visible in darkness? My first thought was no, because spells don't do more than their descriptions says they do and Guiding Bolt doesn't reveal invisible creatures in general, but the spell explicitly said "dim light" "on the target" which the ranger would be visible in, so I'm not sure.
The Gloom Stalker's Umbral Sight ability:
At 3rd level, you gain darkvision out to a range of 60 feet. If you already have darkvision from your race, its range increases by 30 feet.
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.