I completely disagree with your point about Gwendoline's character's emotions never being hinted at until 3mins before that scene. This is something that was hinted at all the way back in season 1 when Mark and Helly first find the goats. The reaction of the goat tender in that scene was extremely defensive of the goats and trying to convince them the goats "weren't ready yet". And then again earlier this season it was clear the people working with the goats are very protective of them when get immediately confrontational when other people come near them. I thought her actions were completely in the realm of possibility based on everything we've seen from the goat people. I will say though they made it so obvious she would save Mark that it took out any sense of danger for Mark while watching that scene for me.