The hunters of Bloodborne are the backbone of the society in Yharnam. They slow down the spread of the scourge while protecting the people from the beasts already in their midst. The job is mostly thankless since the citizens of Yharnam blame the hunters for spreading the scourge, even though they've done their best to prevent just that.
Gehrman is the first hunter charged with the task of killing the beasts in the night and presiding over the Hunter's Dream. He's the guiding force and mentor of the player character, who points them in the right direction throughout the story until the very end. His personal story is quite tragic and plagued with pain and suffering, but he puts on a smile for the hunters in his dream.
A Hunter's Job
Gehrman's job was to kill beasts in the cover of darkness so townsfolk wouldn't know the blood they consumed was turning them into the beasts they feared. There were other hunters that served the Church and the School of Mensis but Gehrman's hunters were the only ones with access to the Hunter's Dream and workshop. It's not totally clear how the Hunters came to be, but Gehrman was the very first as his title implies.
Gehrman mentions the names of higher-ups in the church who were in charge of controlling the beasts and the spread of the scourge, indicating that he knew them personally. Taking charge of killing the beasts himself must have been a huge burden to shoulder, not including all the baggage that comes with controlling the dream.
Relationship To Other Hunters
Throughout Bloodborne, players come in contact with other hunters who have been on the hunt much longer than they have. It's safe to say that not all of them have met Gehrman or went through the dream because some hunters worked for different factions of the church or never left the waking world.
What's interesting is that multiple hunters mention the dream and even Gehrman's doll, indicating that they've met and received guidance from the first hunter himself. This begs the question: how did they escape the dream with no consequences? And if there were consequences, what were they? There's so much mystery behind Gehrman's true motivations and what the dream exists to accomplish that these other hunters just make the mystery run deeper.
The Workshop
The workshop was a branch divided up from the Church of blood ministration like the school of Mensis and the Choir. Each had their own role to fulfill when it came to the old blood and managing the beasts that plagued Yharnam. After the Church decided that the workshop served its purpose and hunters from other factions could take care of Gehrman's work, he abandoned the waking world and turned to the dream for purpose.
After that, Gehrman called to the Moon Presence to give him power in the dream, and a place for hunters to continue their duty without the support of the church or any schools. In order to continue working in his workshop, Gehrman essentially traded his soul to the Moon Presence; consequently, he couldn't return to the waking world.
The Hunter's Dream
Gehrman guides the player through the dream and through Yharnam using the lamp posts and his words. The main character doesn't even question the dream, even though it allows them to die and return countless times at the cost of their sanity. According to the Third Umbilical Cord item, the Hunter's Dream was brought to existence by the child of a Great One who precipitated the encounter with the pale moon.
It's difficult to understand why the dream took the shape of Gehrman's abandoned workshop in Yharnam, but it probably has to do with how his own thoughts manifested themselves. With Gehrman's desire to continue the hunt and prevent the scourge of the beasts, the Hunter's Dream took a shape familiar to him as the first hunter.
The Doll
In the Hunter's Workshop of the waking world, players can find a doll that looks exactly like the doll in the Hunter's Dream. This doll wears a small hair ornament that the player can take and give to the doll in the dream. When she receives the hair ornament, she sheds a single tear that can be used as a healing item.
The interesting part of the tear is its item description which says that the creator of the doll possibly just wanted a friend, albeit in vain. Gehrman's timeless isolation in the dream is clear from his decrepit body and raspy voice, but the doll being around in both the dream and the waking workshop goes to show that his loneliness has been around longer than he lets on.
Secret Dialogue
There's a chance to find Gehrman in the garden of the Hunter's Dream asleep and muttering to himself. This rare opportunity offers a unique insight into the isolation and imprisonment Gehrman has suffered through and how he feels about it. He mutters to his masters and predecessors, or anyone who hears him, to unshackle him from the dream.
The Hunter's Dream was the answer to Gehrman's prayers to the Great One, but it ended up becoming his undoing, a prison for him to spend eternity in with no escape. It's no wonder he's driven to madness after surviving the long hunt as well as watching other hunters lose themselves to madness before he did. The torture is excruciating and lonely, anyone would go mad under circumstances like these.
Ending The Night
After killing Mergo's Wet Nurse, the player returns to the Hunter's Dream only to find the workshop burning down. It's not totally clear, but this seems to be because the purpose of the workshop was to facilitate killing a Great One. Once the player slays a Great One and the Nightmare is ended, the Dream has served its purpose and goes up in flames.
Gehrman seems happy that the night can finally end, but it's questionable if he will even get to leave the dream after the player chooses an ending. He poses a question to the player, about if they will accept their death, separating them from the dream, or if they will reject the proposal. Depending on the answer, Gehrman calls the player mad, as if he lost his own sense of self in the process.
The Fight
If the player decides to reject Gehrman's proposal, he will call them mad, and say he will end their dream himself. He uses his iconic scythe to exploit its long-range against the player's limited reach. The best way to deal with Gehrman is to actually parry him since it's possible to parry almost all his attacks. The horizontal swings of his scythe are the easiest attacks to parry.
When Gehrman switches to his sword and blunderbuss weapons, it's important to use mobility to bait out and avoid his attacks. Staying close to him while he has these weapons out may seem impractical, but it actually gives the player the best chance of baiting out his weaker moves that pose little threat. Weaving in and out of his range keeps him controlled and easier to deal with.