Yes. On iOS you download them from Google Photos first; on Android you can pick them from Google Photos when adding a snippet.
On iOS
The iOS app can't pick from Google Photos directly.
To use something from Google Photos, open Google Photos and tap Download. Once downloaded, it's in your library when you choose content in 1SE.
On Android
Google Photos works as a picker: you choose the photos or videos you want each time, and 1SE only sees what you picked. 1SE can't browse your whole Google Photos library, and the picker groups your media by month rather than by individual day.
To browse day by day, pick from your phone's Gallery instead.
If a video isn't on the phone, download it from Google Photos to your device, then add it in 1SE from your library.
Auto-Fill reads what's stored on your phone, so it isn't affected.
