Depends on what you want to do. Photos will show BOTH the pictures in your phone AND those stored in the cloud only. Now, the Gallery app is not a 'place', it's just a 'window' to the media files in your phone; this should show the same LOCAL pictures and videos as Photos does, but it will not display the cloud ones because there is no direct integration between the Gallery app and Google Photos, unfortunately.
Now, if what you want is to keep pictures you have in your Photos account but are only stored in the cloud (which would not show in your Gallery app), then you need to save those pictures and store them in your phone; only then will those also show up in Gallery. To do that, go to your Photos app, select a picture that's on the cloud only, and when viewing that picture tap the menu button (three dots) on the top right corner. From there, select Download and boom! Now the picture will be saved to your device and, hence, be accessible through the Gallery app as well!