user@rpi5:~/Downloads $ flatpak install gossip-0.10.1.flatpak

Note that the directories

'/var/lib/flatpak/exports/share'
'/home/user/.local/share/flatpak/exports/share'

are not in the search path set by the XDG_DATA_DIRS environment variable, so
applications installed by Flatpak may not appear on your desktop until the
session is restarted.

error: The application com.mikedilger.gossip/x86_64/master requires the runtime org.freedesktop.Platform/x86_64/23.08 which was not found