This is for iOS users. Do this if you have a video saved in Google Drive that you want to save in the Photos app.
1. Open the Google Drive app
2. Tap the ellipsis or the three horizontal dot icon along the filename
3. Select “Send a copy”
4. Tap “Save Video”. Depending on the file size and your connection speed, it may take a while to download.
*Works on iOS 12.1 and Google Drive app 4.2018.44203