If I'm understanding it correctly,
/usr/lib/environment.d/nix-daemon.conf sets $PATH, but then
/usr/bin/gnome-session starts a login shell, which sources /etc/profile
on startup, which resets $PATH to a default value.

Related: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=869144

Would it make sense for nix-setup-systemd to set $PATH somewhere else?
Or maybe the default /etc/profile should be changed?

my login shell is zsh and im affected by the same bug, if I change
~/.zshenv to export the variables set by /usr/lib/environment.d/nix-daemon.conf
i can see the variables being exported to my env (i3, zsh, stterm, etc).

so i think the bug is more general on how debian respects (or not) what's set by systemd's environment.d directories