No idea what floorp is, but it sounds like it just messed up your desktop cache. I doubt it actually erased anything. Unless you clicked through the ‘you’re about to uninstall 530 packages’ popup that you get when you break your OS by installing from third party repositories that don’t support your OS version, then it’ll uninstall every conflicting package (which in some extreme cases means “your desktop environment”). Your user folder should still have all of your files though.
Also, if you enable the AUR on Endeavour, make sure to always update every package every time, or the same thing can happen there without adding any other software sources.
can anyone tell me how to theme GTK-based GNOME apps on KDE
Badly. Neither KDE nor Gnome can style the other particularly well. KDE offers more options, but I’ve never bothered with trying to get the theme to feel “right” on the KDE installs I’ve messed with.
You can do basic styling in the main UI settings. I don’t think KDE supports styling libadwaita yet, though. I also believe you need to do some extra hackery to make Flatpak apps pick up your KDE settings. You may also need extra packages (qt5-style-plugins or such) depending on the applications you install.