In a nutshell:
> which gitmux && ls -al (which gitmux) && gitmux
/home/psychoslave/bin/gitmux
-rwxrwxr-x. 1 psychoslave psychoslave 2752512 6 janv. 19:11 /home/psychoslave/bin/gitmux*
fish: gitmux: command not found...
> echo $PATH
/home/psychoslave/.rvm/gems/ruby-2.7.0/bin ~/bin […]
So, what's going on here? Fish clearly know where to find the program, but will pretend it doesn't?