Setting environment variables in Gnome session

[Jonathan Frederickson]

I'm trying to install some software through Flatpak alongside software
installed through Guix (on a Guix System install) and I'm running into
what feels like it should be a minor issue. On other distros (including
my desktop where I'm running Guix as a foreign package manager), I
would modify XDG_DATA_DIRS in $HOME/.profile to accomplish this.

However, my Gnome session in Guix System seems to ignore this file. 
I've tried creating a file in my home directory in /etc/profile like
so, and as far as I can tell it's never getting run:

export XDG_DATA_DIRS=$XDG_DATA_DIRS:/var/lib/flatpak/exports/share
echo "hi there!" > $HOME/test.txt

Is there a preferred way to set environment variables in a graphical
session?