What happened after she got bitten by a snake?

Ty's place in the desert is absolutely crawling with snakes. He tries to reassure Gemma that they're "pretty much" harmless. But bearing in mind that Ty's someone who's actually kidnapped Gemma, we can be forgiven for not trusting him. Besides, we later discover that Ty's been collecting snakes so that he can prepare anti-venom. There'd be point in doing this if the snakes weren't poisonous.
At the point in the story where Gemma gets bitten, she's come to develop an emotional attachment to Ty. She doesn't know it, but it's likely that she's experiencing a form of Stockholm Syndrome, a psychological disorder whereby someone identifies with their captor. So when Ty leaves her a note telling her that he's gone out, instead of trying to escape, Gemma goes looking for him.
That's when she gets bitten by a snake, and it happens to be a deadly poisonous one at that. Ty attempts to treat Gemma using some of the anti-venom he's been collecting, but it's no good. He has no choice but to take Gemma back to civilization, to a hospital where she can receive treatment for the snake bite. Thankfully, she pulls through. And it's at the hospital that she finally realizes the full scale of the physical and mental ordeal she's been put through by Ty.