1. "before gravity would take its inevitable effect" - I would not say it is slang but it refers to gravity 'taking effect' on her face i.e. getting older and her skin sagging and her becoming less attractive.
2. “You’ve put me in a tent? At a festival? What the hell?” Sam Redwood was not a happy man.
3. “No shit, Sherlock. Look at that Don, even the old bald guys are getting laid and I’m stuck in a tent.
A tent means a tent in this context I think. At music festivals, people sleep in tents in camping grounds. 'You've put me in a tent' means that the person has assigned him a tent/not provided somewhere else for him. 'Stuck...' means that he has no other option available to him.