Sharpshooter is considered to be among the most overpowered feats in D&D 5e. Its gross damage boost is even more pronounced when one has a consistent source for Advantage.

Gloom Stalker has the following feature at level 3:

UMBRAL SIGHT (…) While in darkness, you are invisible to any creature that relies on darkvision to see you in that darkness.

Fine in a city or wilderness adventure, where it's only dark occasionally, but in dungeon adventures, darkness is prevalent. And when the campaign is a megadungeon campaign, that is dungeon all the time. Even if there is light somewhere, it is sufficient that the Gloom Stalker shoots out of an area of darkness to gain the benefit of invisibility. It is nearly like having permanent Greater Invisibility going.

Being invisible grants Advantage. So nearly all of the time, these Sharpshooter attacks are made with Advantage. Together with Archery, this effectively negates the -5 penalty to attack rolls from Sharpshooter.

Gloom Stalker at 3rd level also gets Dread Ambusher, which adds his wisdom bonus to Initiative, and gives an additional attack with +d8 damage if he attacks in the first round. As a Ranger, at 5th level he gets yet another Extra Attack.

Our situation

We are playing Dungeon of the Mad Mage, so we are in dark dungeons a lot of the time. Our party has three characters of 12 level (currently), all with darkvision, a plain vanilla Wizard 12, a Paladin 6/Sorcerer 6, and the Gloom Stalker, a Ranger 6/Rogue 5/Cleric 1.

He took a level of War Domain Cleric, for yet another set of Extra Attacks and Bless to further boost his hit rate, and a few levels of Rogue for Sneak Attack damage. We often Haste him, as he has the highest damage output per attack, even higher than our Paladin.

I think we all have competent character builds, but in the face of a typical combat opening to a flurry of 5 attacks with Advantage, each dealing 20 damage or so on average, few opponents live to see the end of their second round. Nevermind if he rolls a critical, which, given the many attacks, is not that rare. I like it, it makes our life easy. But I pity our poor GM who has to put up with this.

Issues this has been causing

• The brutal effectiveness of the Gloom Stalker invalidates many fun, but less effective tactics. It is just not worth the effort to try something else when he just can mow down everything.
• Our GM likes to play by rules as written, but has taken to rulings that often feel biased, in an attempt to keep the game challenging and make things more difficult. (We are only 3 PCs, instead of 4, but deal with all the same challenges).
• For example, originally in the campaign it was very hard for us to detect and pinpoint invisible enemies. When the Gloom Stalker came into swing, invisibility suddenly was reduced to the pure mechanistic minimal effect (after heavily consulting this site as to what that would be :)).
• The GM has tried other things like Faerie Fire by the drow, but the accessibility of Haste and high mobility of the rogue with Cunning Action make it easy to evade light areas, and even allow for a Hide action every round invisibly. The long range of the longbow allows the Gloom Stalker to make full use of his mobility and shoot into light areas from the dark. It even may harm the opponents, as he can see into the lighted area from further out than his darkvision's reach.
• The GM regularly coordinates the intelligent monsters to "monster-ball" us with most of the monsters on a level in one combat to overload our kill rate. We had that against the drow twice, and against the nagas and bulliwugs. We still won one of these, and took a tactical retreat after causing severe losses in the other two. When I tally these fights on Kobold Fight Club, they typically end up with 5-10 times the XP worth of what would constitute a "deadly" encounter for four characters of our level.

Is the Gloom Stalker overpowered?