This app was mentioned in 5 comments, with an average of 1.40 upvotes
It's not Google Photos but, I'm using the Photo Guru app(https://play.google.com/store/apps/details?id=com.cmpsoft.MediaBrowser) on my Nexus Player until there is an official app.
Folder system isn't the best but, once you work it out, you can look at your photos from Google Photos (known in the app as Picasa) and Google Drive as well as some other sources.
I use PhotoGuru to view my Google Photos on the TV and it can set them as a screensaver if your TV is capable of using third-party screensavers. My Hisense is not. PhotoGuru is not free, I think the Google Photos addon is $2 and the screensaver addon is another $2.
https://play.google.com/store/apps/details?id=com.cmpsoft.MediaBrowser&hl=en_US&gl=US
PhotoGuru works well for me (https://play.google.com/store/apps/details?id=com.cmpsoft.MediaBrowser) and supports multitude of sources, although you have to buy each source separately. It doesn't hook into ambient mode, but CCwGTV supports screensavers, which replace ambient, and work fine.
I like PhotoGuru. It can also display photos from my DLNA server. https://play.google.com/store/apps/details?id=com.cmpsoft.MediaBrowser
If you want a native app look no further than https://play.google.com/store/apps/details?id=com.cmpsoft.MediaBrowser Work in pure Android and LeanBack - accesses most sources and allows for good browsing. Nice bonus is that it can act as a dreamtime source..