I found a way to display Google Photos, however, there is a problem.
According to your Knowledge Base Google Photos does not have an API for 3rd parties. You do support downloads from Google Drive and one of the folders in Google Drive is Google Photos, which not surprisingly contains all your Google Photos. In my PixStar account I connected to Google Drive and the Google Photos folder and syncing to my Frame started. Everything seemed OK until I noticed that pictures in Portrait mode were being displayed in Landscape mode instead. I downloaded one of the pictures that was displayed incorrectly and it displayed correctly on my PC, i.e., the Rotation metadata is correct. I sent the downloaded picture to the frame and it displayed CORRECTLY! It seems that when you get a Google Photo from Google Drive that the metadata is incorrect or you ignore the metadata. If the problem is on your side (which makes the most sense) it should be easy to fix and then you will have Google Photo compatibility without having to implement a new API.