Finally! My transition to Linux is complete! For anyone else who is a bit of a Linux donkey like me that couldn't find where to put their WAD files, literally paste the location mentioned in the installation notes ~/.var/app/org.zdoom.GZDoom/.config/gzdoom into your file browser and it takes you straight there! Thank you so much for this bit of software!

Error msg: " Invalid url: 'apt://org.zdoom.GZDoom' given, exiting " then below in the same msg, "Invalid package name 'org.zdoom.GZDoom' ". Ergo, it didn't install or run. Thanks!

Hexen does not work on current flatpak version g4.8.2-m. No issues on g4.8.2 deb from their site