Here is what I did that I got this message. I keep updating things to latest so this is in respect to 16-July-2023.
$ sudo flatpak update
Looking for updates…
Info: runtime org.gnome.Platform branch 41 is end-of-life, with reason:
The GNOME 41 runtime is no longer supported as of September 17, 2022. Please ask your application developer to migrate to a supported platform.
Info: applications using this runtime:
org.gnome.gitg
Nothing to do.
$
I would suggest removing it, and installing the one from the Fedora Flathub repository instead. Even if the application is not updated, we rebuild these for every release, using our newer supported runtime (and, often, increased security hardening and other improvements).
I would suggest updating. That runtime is no longer getting security updates, and by its nature gitg operates with remote data, making it potentially vulnerable to exploits.