what does the NG+ do? never tried it.
Gwen resets time to Day 1. So the game basically restarts -- but:
Your character keeps his/her money (+ the income earned through microtransactions), his/her level and all acquired skills (e.g. the knowledge of how to do pain/sperm sluts, the ability to locate each person's location via cell phone, etc. ). You also have the room keys you purchased and your inventory.
And you can decide whether Gwen will also be reset to the beginning or whether you will take her with you into this new timeline and the relationship you have with her (she will then be a powerful ally to persuade the other girls).
A way to play to take advantage of this would be, for example, to change gender before resetting and then restart using Gwen. The genders play a little differently when flirting with the ladies.