If you have a computer, go to photos.google.com, select the photos that do not already reside on your device, and then go to the menu and select download or use ctl+D to download. This will put the photos in the download folder on your computer, which you can then transfer to your DCIM folder on your device and it will bring them up in the gallery.
If you try to do it from the Google Photos app, you will have to select and download each photo individually (not a big deal if you have only a few photos), but you will only get the download option on photos not already residing on your device. Since Google Photos is essentially a backup application/photo viewer, if you had photos you deleted from your gallery, they would still be available in Google Photos, but if you deleted them from Google Photos (backup), they are gone from your device and from Google.
Or, you can factory reset your phone, and when you start up the phone and sign into your Google account, you will be prompted to restore your Google backup, which will contain all your photos; then, you can sign into your Samsung account and restore your Samsung backup. You can only restore the Google backup upon first starting up after a factory reset -- which is probably the most moronic thing for a backup service.
