The widescreen file game_widescreen_gfx.pk3 is not found outside the directory in which gzdoom executable file is located.
This happens on Linux Mint 20 (based on Ubuntu 20.04) with a newly installed, from the Downloads page, of GZDoom for Ubuntu. It also happens when I build GZDoom locally on this system and use that instead, both for master and stable builds.
Steps to reproduce (from bash prompt):
Issue the command gzdoom -iwad doom at the bash prompt from outside the executable file's directory but with gzdoom in the path.
GZDoom will fail to find the widescreen assets .pk3.
Issue the command /path/to/gzdoom/executable/file/directory/gzdoom -iwad doom at the bash prompt from outside the executable file's directory.
GZDoom will fail to find the widescreen assets .pk3.
Issue the command gzdoom -iwad doom at the bash prompt from inside the executable file's directory.
GZDoom will find and use the widescreen assets .pk3.
This does not affect other files normally located in basedir, such as gzdoom.pk3, only the widescreen assets file.