How to find location of another phone with out having access to it.

[Andrea Reyes]

I thing my gf took one of my phones and downloaded the back up to it.

 

[B. Diddy]

Welcome to Android Central! If it's your phone, then your Google account should still be on it, and you can locate it using Google's Find My Device: https://support.google.com/android/answer/6160491?hl=en

If they somehow were able to reset your phone and set it up as their own, then you won't be able to locate the phone (unless you know how to log into your girlfriend's Google acount). It also means you didn't have adequate security on it (i.e., a secure screen lock that you didn't share with your girlfriend) -- unfortunately, that's one of the consequences of not having good security on your phone.