Oh, Gabrielle, you rebel turned refugee turned super-zombie… who never says a single word out loud to Mary. So why is she such a big deal to the Marymeister?
We know Gabrielle was a rebel—and probably a dreamer—because she told Travis that she visited the ocean once. Since that involves running through the Forest of Hands and Teeth and surviving—both there and back—we know she had to be both tough and brave. And willing to leave her village.
Even though Mary doesn't find out for awhile that Gabrielle has been to the ocean, she's intrigued by someone from the Outside. We already know Mary is obsessed with the ocean; seeing Gabrielle just fuels her hopes of an Outside world beyond the fences.
Gabrielle is also a refugee. Mary sees her wearing a bright red sweater and crossing the snow from the path late one night, and later on when Mary and the crew encounter a village overrun by zombies along that same path, she realizes Gabrielle must have escaped the attack.
Finding out about Gabrielle's ruined village both lifts and drops Mary's hopes. The lift? She realizes there are other villages out there. Gabrielle really did grow up in another place, with different rules and traditions. The drop? Gabrielle is just an escapee… she wasn't trying to send a message or make an alliance or anything. She just wanted to survive.
Her whole village succumbing to zombies is just the beginning of Gabrielle's troubles. Sister Tabitha ends up feeding her to the bloodsuckers, but instead of turning into just a regular old zombie, Gabby mutates into a super strong and super vicious super-zombie. Which is, you know, just super. She leads the attack on Mary's village, though we don't know if it was because of her bloodthirsty zombie instincts or to exact revenge for Sister T ruining her life.
Ultimately, we think that it's the Sister T connection that most draws Mary to Gabrielle. Seeing her as a super-zombie terrifies Mary, and she realizes that she could have become just like Gabrielle if Sister Tabitha had made a different decision about her fate.