Walkthrough:
Alternative ending: Choose the option "Shoot Jamal"
You need to have sufficiently low corruption by Jamal to achieve this, going with Dredds route in the strip club lowers corruption, as does not going along with the most extreme actions (sending selfies, keeps sucking while talking on phone in the van, having sex on the plane, etc.).
SavannahxMC route: You need to have chosen her route in the library and not be in a relationship with Astrid.
GretchenxMC: Choose “I want to stay with you mom” You need to have chosen her as your potential love interest when she asks you in the beginning at the restaurant. You must also have sufficiently high attraction between you (not all are necessary but choose actions such as flattering her, be supportive when she asks if she should introduce Savannah to the studio, do drugs rather than trying russian roulette, tell her about it, stand up for her when Monty abuses her etc.) You must also be a virgin, meaning you can not have sex with Maggie, not have a relationship with Astrid or be engaged with Khalida.
How to Intimidate Savannah: You need to have sufficiently high Rage some but not all of the following are necessary: Go along with the actions of the Cloveside Killer when he chats with you, don’t dismiss him in the first chat, Threaten him in the second chat, say you enjoyed killing Sherman in the third, punch Astrids father and fight back against Garvey.