Gwen's "new game plus mode" is her ability in the Prime mod.
It gives you the option to restart the game from day 1. The OPPORTUNITY, not the compulsion. You can reject her first offer and approach her again at any time, days, months, years later.
Talk with her / Relationship status / About that talk we had before... / About that New Game Plus time travel thing...
You're practically starting a new game, but:
You keep your money, your items, your skills acquired from the girls (e.g. from Kitty that you can see where the girls are), your level (for example, this makes an impression on Emma when you first meet (something like: Zero your school achievements are impressive)),
and you can decide whether Gwen remembers you. If so, you'll take a trained slave, *errr*, I mean a fully-fledged girlfriend into the next round who will be an invaluable help in the next game round. For example, if you have sex with Gwen in public in front of one of the other girls. This either totally embarrasses the other Girl or encourages her to join in. Even though she wouldn't want to have sex with you otherwise.