Future Press game guide description: "Gilbert is a foreigner to Yharnam afflicted with the Scourge who traveled to the city in search of a cure. This being the case, he sympathizes with your situation from the start and does his best to help you in spite of his worsening condition; by the time you first meet him he's already incapacitated with no hope of survival, and can only provide you with information."
Gilbert is a foreigner to Yharnam, just like the player. Because they are both outsiders, Gilbert is sympathetic to the player’s cause. Gilbert came to Yharnam in search of a cure to his ailment. It is likely that once in Yharnam he met with one of the Church’s blood ministers and received treatment. Perhaps this minister for the Healing Church gave Gilbert a Flamesprayer, an item he will give to you once he is sure he will not live through the night. Gilbert will say to you “But before I… Take this…(gives you the Flamesprayer) I made no use of it.” That sentence “I made no use of it” implies that he was given it to make use of by someone, and considering the origins of the Flamesprayers, it was likely a member of the Healing Church; a blood minister.
Gilbert is quite knowledgeable about the Yharnam area for someone who is a foreigner to Yharnam. He is familiar with central Yharnam and the passage to the Cathedral Ward. Evidently he may have made the journey to the grand cathedral deep within the Cathedral Ward himself, perhaps through the “colorful" aqueduct area, since the Church would never let an outsider near the place if they could help it and Gilbert has been treated by Yharnam’s “strange blood.” Outside of this knowledge about the Healing Church and Cathedral Ward, Gilbert knows little.
What afflicts Gilbert is incurable, but the strange blood ministered to him in Yharnam gave him time. By the time the player encounters him he will tell them he can’t stand even if he wanted to, and we can hear that he is constantly beset by a hacking cough. The effects of his illness have caught up with him.
Gilbert is inflicted with the terrible ashen blood ailment, a disease that ravaged Old Yharnam long ago. Ashen blood is, as Gilbert tells you, incurable. The antidotal tablets that were created to counteract the disease will only provide short-time relief, the disease eventually overcomes its victim. But just like those afflicted in Old Yharnam, the disease can be treated with Yharnam blood, and again, just like those in Old Yharnam, the blood will transform them into a beast.
Gilbert appears blissfully ignorant of the darker truth behind the blood of the Healing Church. He will tell you: “Their strange blood bought me time. I was most fortunate. Unharmed by the plague of beasts, I can even die human” not realising, just like other foreigners before him, that the plague of the beasts and the cure of blood are symbiotic, one causing the other. Just like the victims of Old Yharnam, Gilbert will eventually succumb to the scourge of beasts and transform. We find him after the Blood Moon rises, in the streets just outside his house. The bars previously protecting him from any beasts that may have entered his home, are bent outwards, indicating they were broken from the inside. On Gilbert’s corpse we find the Clawmark rune. The rune describes an impulse to seek the warmth of blood like a beast, an instinct deep within the hearts of men…