There's a scene in "The Color Purple" where Miss Celie tells, Shug - "He beats me, because I'm not you." I believe this is the crux of this situation. For close to 7 years, Brian, the self-absorbed, isolationist narcissist tried to change the all giving and nurturing Gabby into his doppelganger. He wanted a female Brian, a hermit with a useful vagina.
But, she couldn't chuck her free spirit, love of people and desire to teach the world about caring for the world. He was as small as a cancer cell. She was as large as the universe. He could not beat her embrace of the world out of her. He could not spirit her far enough away to cut her ties to the world she truly cared to nurture. He could not brow beat her into the kind of devotion that expects nothing in return.
He wanted to flee from the world. She wanted to change the world. He wanted to abandon all ties. She wanted to share her embrace with everyone.
Gabby said no too many times to a person who tried to groom her into believing a life in handcuffs was a life liberated simply because he was doing the handcuffing. In the end, she was a failed experiment, an incendiary catalyst that ignited the rage at the heart of his iconoclasm. Like a gladiator, this happy, light-hearted, little girl withstood his attempt at brainwashing for 7 years in order to lighten his heart, heal his soul and make him fall back in love with the entirety of the world - not just the rocks, streams and mountains upon it.
In the end, his sickness proved to be both incurable and deadly.
JMHO