Gnome's GDM (Gnome Display Manager) manages graphical display servers and handles graphical user logins. See this page on the ArchWiki for more information.
(from the above link): To autorun programs: Ensure your `~/.xinitrc` or `/etc/X11/xinit/xinitrc` files source `~/.xprofile`:
#!/bin/sh
# Make sure this is before the 'exec' command or it won't be sourced.
[ -f /etc/xprofile ] && source /etc/xprofile
[ -f ~/.xprofile ] && source ~/.xprofile
Then, create your `~/.xprofile`, and add the commands you want to use when you start your GUI login session.