Installing stable builds

$ flatpak remote-add --from flathub https://flathub.org/repo/flathub.flatpakrepo
$ flatpak install flathub org.gnome.Photos

Installing nightly builds

$ flatpak remote-add --from gnome-nightly https://sdk.gnome.org/gnome-nightly.flatpakrepo
$ flatpak remote-add --from gnome-apps-nightly https://sdk.gnome.org/gnome-apps-nightly.flatpakrepo
$ flatpak install gnome-apps-nightly org.gnome.Photos