Follow these simple steps to start using Flatpak
Flatpak can be installed on Gentoo using the Flatpak overlay. See the readme for instructions.
Flathub is the best place to get Flatpak apps. To enable it, run:
$ flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
To complete setup, restart your system. Now all you have to do is install some apps!
Note: graphical installation of Flatpak apps may not be possible with Gentoo.