As their name suggests, the Gentlemen are a group of people with refined class and taste. They are also handpicked from every dominant species in the galaxy to represent and defend their world.
First mentioned by The Gentle Coltte of Leisure(Equestria's Gentleman), they devote their lives to "Keeping the Balance." This often relates to the abuse of Magical power, and the outbreak of Eldritch Terrors. However, it should be noted, that they do not destroy evil. It is believed by the Gentleman that "Destroying Evil only creates more Evil in it's place." and that "Good can only triumph for so long". Relating to their theme of balance.
Their base of operations is Atlas Omega. It was chosen because of it's "Vicinity to the various worlds under Gentleman jurisdiction" and it's "Vast liquid resources"
While it would appear that everyone is referred to as "Mister" or "Madame" while in the ranks of the Gentlmen, some appear to have more "Swing" in the organization( Gru'uhnd) most notable. Though it is most likely that more famous members, or members who have accomplished greater things, are more respected and therefore have more influence.
There is also a "Council" comprised of higher ups. They are apparently chosen by vote, and presumably stay in their seats of power until death. They were killed by Gru'uhnd during the Gentleman arc, and subsequently replaced. Gentle Coltte was considered for a seat, but did not make it due to his former criminal status.
Given the dangerous nature of their work, the Gentlemen need to be properly equipped to fight anything and everything. These abilities vary from Member to Member. Mr. Gru'uhnd, for instance, is adept at pickpocketing(Stealing from Gamzee mid-battle) and Mr. Qin possesses enough balance to stand atop one of Rainbow Dash's wings as she flew. It is assumed that each Gentlemen is an incredibly strong individual, possibly the most capable of their worlds. They are never to be taken lightly.
The Gentle Coltte had a manual in his manor that contained a list of several hundred creatures, demons, elder gods and time paradoxes that relate to their Organization. If these manuals are standard issue, then it would be assumed most Gentlemen are very knowledgable of the supernatural world.
Both Mr. Gru'uhnd and The Gentle Coltte were shown to have "Magic Dampening Tattoos" on their body. These absorbed and negated most magics, including ones that would have otherwise helped them, like Trixie's magic. It is unknown how many Gentlemen have these, but considering Mr.Gru'uhnd's comment about "Not getting far without them in [The Gentlemen's] world", they may also be a mandatory part of Gentlemanhood.
The Gentlemen are comprised of many different species, each one apparently represents their homeworld and is a member of their dominant species. Members(Former and Current) include:
The Gentle Coltte of Leisure: Representative of Ponyville. See his main article for more details.
King Blueblood the 1st.: Former Gentleman of Ponyville. He passed his title onto The Gentle Coltte before he died of an illness. His son, Prince Blueblood, was meant to take his spot, but he was too young at the time.
Mr. Gru'uhnd: From an Unknown World. His species is Gorilla-like. Currently Incarcerated in "The End" after his plot to take over the Gentlemen. A formidable foe and mastermind of the plot that captured our heroes and placed them on Atlas Omega. (Should be noted that due to 'time and paradox shit', The Midnight Crew somehow ended up with a Gru'uhnd after the battle, who was promptly and accidently eaten by Hearts Boxcars, so really should just be disregarded.)
Mr. G'lysph: A Horror-Terror Gentleman. He can apparently shapeshift, and assault people's minds. He proved nearly impossible to defeat, because his abilities allowed him to blind and terrify anyone who came close.. He was defeated using the Crystal Pony's Song, and imprisoned in a runed Birdcage by Mr. Maccaw.
Mr. Rulek: A "Sandstone Construct". He is apparently in control of the Judgement Grounds, because he can remotely shift it. He was "Killed" when Equius pounded his stone body to pieces. He finds the term "Golem" deragatory.
Mr. Ssssvilk: A Snake man with an appetite for fine food and fine women, both of which he swallows whole.. A lech and in charge of The Gentleman's Pleasure Palace, he was killed when his latest meal, Terezi, burst out of his stomach.
Mr. Ophiuchus Morres: The former Alternian Gentleman. He was killed by Mr. Gru'uhnd in his power hungry trip, immediately after they defeated "The Corrupted One". Spent his last hours taking care of the Gentle Coltte, and as such GC looks up to him.
Madame Kull: Insectoid Arena Mistress and ruler of The Judgement Grounds. She was killed when Mr. Rulek flooded the arena with Neurotoxin. She couldn't handle the poison, due to her insect physiology. It's possible Mr. Rulek did this on purpose, considering he didn't seem to like her.
Mr. Qin: Chinese Gentleman, and the current Gentleman of Earth. A master at Martial arts, he also seemed adept at using Tavros like a spear.
Mr. Maccaw: A bird... thing. Mr.Maccaw appears to be a robotic, human-like suit, piloted by a Maccaw Bird sitting in a cage where it's head should be. Although intellegent, he's never spoken
Mr.Kumbah: An anthropomorphic lion man. Owner and proprietor of the "Weeping Gallows Insane Asylum" where Gamzee was briefly held. One would assume he would play a greater part in the story, but because of Marceline's interference, his role was cut short.
Mr.Trombone: A Namekian Gentleman. Not much else is known of him, aside from the fact that he appears to be on good terms with Mr. Gru'uhnd. They may even be friends. But that did not stop Mr. Gru'uhnd from poisoning him and all the other Council members.
Mr. Ed, Mr. Warwick, 'Mr. Kringle: Three Gentlemen that appear in The Gentle Coltte's "Hall of Comrades" They are apparently Gentlemen whom he has fought alongside, and the latter may or may not be Santa Claus.
Eridan(?): The Gentle Coltte is currently training Eridan to become the new Alternian Gentleman, but it is unknown how this is progressing. Eridan deeply wishes to live up to Ophiuchus' example.