Ya Im not aware of a way to get all your Google photos into Gallery... As stated already, use the Photos app which should be pre-installed on your phone. I like how gallery categorizes photos (camera, wallpaper, music, FB, etc), but I was bummed too when I opened Gallery and found none of my photos. But Photos is a nice little app once you get used to it!
When I set up my phone, I had forgotten to back up my Gallery. Fortunately, 99.5% in the Photo app. Is there a way to migrate to the Gallery? I believe that the Photo app can be almost immediately.
When you download the Google Photos app, if you select a photo (yes one at a time) and tap the 3 dots in the upper right, you can download it (one at a time...{why Google why?}) or set it as a contact photo or as a wallpaper.
I'm not sure why they did this. Maybe Google wants to fulfill their plans of world domination, starting by only using their apps.