Testing on Debian doesn't work
SLADE Version
3.2.8
OS
Linux
Editor
Resource editor, Map editor
Steps to Reproduce and Observed Behavior
Go to test (testing section), set the launcher to GZdoom (downloaded from the web), click Run (with Doom2 as the base resource) and the game does not launch.
Expected Behavior
Launch the mod successfully to test it
Screenshots

Works for me, are you using my deb package or the Flatpak?
Flatpak
@Eonfge I guess this is your ticket then.
From what I understand the Flatpak ships with its own copy of GZDoom and it should be configured to run that although there's a caveat for 3.2.8, but it sounds like it should be setup to just work.
I forgot to mention that I installed GZdoom using Sudo Apt
You can't use the GZDoom version you've installed with apt, since the SLADE flatpak is sandboxed. For testing, GZDoom is bundled and you can start it by configuring gzdoom to point at /app/bin/gzdoom.sh
Did you say that GZdoom is installed on Slade, or am I mistaken?
Yes. If you install SLADE through Flathub, it has a copy of GZDoom included.
I present this similar issue trying to test with Sonic Robo Blast 2. I'm using compiled slade from master branch (updated today), still happens.
Happens when I use SLADE from flatpak too.
I'm using Linux Debian 13 Trixie. this is the message that drops in the terminal.
My settings
@GLideKS Assuming your screen shots are from the self compiled Slade that error indicates the issue is with your srb2 binary not Slade. Specifically it seems like the wrong libmpg123 is being loaded, which the reason for that would be hard for me to guess. I would suggest pointing Slade to a script that writes the output of env somewhere. Compare that to the env of how you run SRB2 outside of Slade. Look for any LD_ variables that might be affecting things. You can also use have that script run ldd on the binary and compare outputs to see what's being loaded.
Based on what Eonfge said it sounds like the Flatpak isn't setup to be able to execute stuff from your host system (seems like a huge short coming to me).
TS is solved