There’s this GLF-OS/modules/default/packages.nix at d7326128d0c8579608a10bb6aa388fb5b9cd7812 · Gaming-Linux-FR/GLF-OS · GitHub
which means you can set glf.environment.edition = "mini"; to avoid discord. but also removes other packages in that list in the link above. and changes many other things.
removing packages from environment.systemPackages is not possible as of yet.
related Removing specific default packages in NixOS
It can only be done via forking glf-os repo, and keeping it up-to-date OR use pkgs.applyPatches and patch glf-os, see for eg. Support flake references to patches · Issue #3920 · NixOS/nix · GitHub, but doing this requires something called IFD which usually slows down nix commands.
or you can try disabling that module and maintain your own packages.nix module like here Removing system packages - #7 by SirGarry
none of these quite beginner friendly.
best make a issue upstream to request a packages.discord.enable option.