~/.bashrc is only sourced if you are effectively using bash as your shell. ~/.profileshould be sourced too under those conditions, but it won’t be unless you add…
source ~/.profile
… to ~/.bashrc or ~/.bash_profile. For some reason, that was not included in the default installation.
However, you are most likely using zsh as your shell, in which case the shell initialization file is ~/.zshrc. But even that won’t be the right place to put that command.
I cannot be sure because I don’t use GNOME, but it is possible that this variable is set from within /etc/environment or some other GUI-related configuration file, in which case there is of course a distinct difference between how X11 is started and how Wayland is started, depending on which of the two you use.
By default should have nothing or close to nothing in it.
It is one of the handful of places to set a global environment variable … but its not GUI-specific or anything … maybe you were thinking of /etc/profile(.d)?
A file under /etc/profile.d/ would have been one of the possible locations, indeed, but /etc/environment was obviously where the variable was set on the OP’s machine.
Or in /etc/profile file itself … my point was environment is not one of the GUI-specific locations for global env vars, unlike profile.
Also OP asked “where to set it” not where the option pre-existed … I dont think manjaro (or many distros) ship with things in /etc/environment. Mine certainly only has what I put in there, otherwise it would only have the original comment in it.