Just my opinion.

I am very conservative about package formats, or managers.
I hate snap, and flatpack is second in the list. There is also AppImage, and I don't like it either..

I would stay in plain RPMs or DEBs.

Well, to my knowledge there are no plans to abandon rpm or deb. In fact, the gretl versions are not directly released in those formats, anyway. That is done by the (excellent) distro packagers, spontaneously the names Dirk Eddelbüttel and Johannes Lips come to mind, and I apologize to those others whom I'm forgetting.

If I'm not mistaken, the release formats from the gretl project in the narrow sense are *nix source tarballs, Windows installers, and MacOS packages (disk images or whatever they're called these days). Any Flatpak or Nix packages would come on top of that, that's why the question is whether it's worth the effort.

cheers

sven