It looks like you need to recover the password first. The Watch is linked to your girlfriend's Apple ID.
If she has forgotten the password then you can recover it here: http://iforgot.apple.com/
Here you will be guided step by step to reset your password.
This article from Apple Support will also help you through the steps and provide additional information in what other ways you can reset your password. If you forgot your Apple ID password - Apple Support
Also below is a link to an article what steps you need to do to give away the Watch:
What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support
I hope this got you started and you can make use of the Watch.