You need 2 PS4 to do this steps in order to activate game sharing:
1. Firstly, buy the game on any account, but for example, let's use your account.
2. After that, log in to the account on your PS4, and download the game.
3. Logout from your PS4.
3. Go to your GF's PS4, and create a user account with your PSN account.
4. Go to Settings > [Account Management] > [Activate as Your Primary PS4] > [Activate].
5. Download the game using your account to your GF's PS4, and then log out.
6. Let your GF login using her user account on her PS4, and now she can play the game.
What you need to remember is DO NOT let your GF login into your account when you're online, since you're going to be booted. So she needs to play using her account.
And if you wanted to share more games, just buy using your account, then login your account on her PS4, download the game, and then logout.