The language is rarely ambiguous as to the nature of Gehrman's obsession portends. Let's look at the item descriptions and use a dictionary to understand one of the words used here:
Doll Set:
"Discarded doll clothing, likely a spare for dress-up."
"A deep love for the doll can be surmised by the fine craftsmanship of this article, and the care with which it was kept."
"It borderlines on mania, and exudes a slight warmth."
Maria Hunter Set, final sentence:
"Maria is distantly related to the undead queen, but had great admiration for Gehrman, unaware of his curious mania"
Tear Blood Gem:
"Created from a shining silver doll tear, this blood gem is a quiet but unfaltering friend that continually restores HP, the life essence of a hunter. Perhaps the doll's creator had wished for just such a friend, albeit in vain."
Mania is a word that repeats often to describe the relationship between Gehrman and Maria. If you don't know what the word mania means, by use of Merriam Webster and how it relates to this context:
2 a : excessive or unreasonable enthusiasm <a mania for saving things> —often used in combination b : the object of such enthusiasm
I think that while their working/professional relationship likely resembles something akin to mentorship, the nature of Gehrman's fondness for Maria stems from a man who is past his prime and in love with a younger woman. While we aren't ever shown explicitly what he feels about the relationship, we see signs of repression and guilt emerge from Gehrman himself. He makes a literal object/effigy of Maria, the item narration describes the nature of Gehrman wanting an "unfaltering friend", to be unfaltering in another way is to be "loyal." Which would suggest a bond of sorts, I think to argue whether or not that is romantic in nature is probably asinine.
He loved Maria, but he didn't know how to express that to her and so he did something that we see many people do in modern art. He created something that would resemble her, it's something by design of the creator that would love in a way he thought he would want. By the time we see Gehrman, clearly whatever the doll is she actually means very little to Gehrman and is, at best, a proxy for the actual object of his desires. Which is to say, his inability to express his feelings plainly to someone he loved is to remove some of the romanticizing we might give Gehrman for not knowing this.
He looks like an old bad ass, and really he is... but there's a human being underneath there. And his profession leaves a social problem unanswered. Was his desire for Maria appropriate? I think we can ask those questions, and should. Certainly making a doll didn't really get him what he actually wanted. Which I think the following line says everything to me about the nature of Gehrman's creation:
"Hunters have told me about the church. About the gods, and their love. But... do the gods love their creations? I am a doll, created by you humans. Would you ever think to love me? Of course... I do love you. Isn't that how you've made me?"
"Isn't that how you've made me?" That's just not much of a love, I'd say. That Doll isn't Maria, and what kind of love is that if it never had a say in the matter?