Story short, I use Google Drive to store my photos, migrated recently from Dropbox. Photos sync works, I can access them in the Cloud and synchronize them to the computer via Google Drive. Yet there is one issue with Windows 8/10. If you put Google Drive to the library or even add it to the Photos app, it will not generate or even bother to index them, not sure if this is Google or Microsoft issue, but fix is quite easy.
Locate your top folder of the Google Drive, right click it and select Properties, then go the the Security tab and add System to it. In order to do that, click Edit… and type System, after it has been added give it Full Control permissions, save it.
Next steps is to go the the first tab which is called General and then click on the Advanced. In there un-check “Allow files in this folder to have contents indexed in addition to file properties”, it will ask to apply to all folders/files and sub-folders do it, once that is done, mark it back again and apply it again.
This will force re-indexation of the items which will fix this issue.
Now I can search my Google Drive and see photos from it in Windows Photos app, including in Windows 10 start menu thumbnail.
PS: this will not force Google Drive re-sync, which is not needed.