You do not start GUI applications in
Those are system daemons that have nothing to do with GUI. If you
want to start an application on start of X session, you have to look at a
manpage for Xsession instead.
For example, my ~/.xsession is
xkbset r rate 250 25
xkbset is run under my user, as if I started it. It
actually has access to DISPLAY and other X resources, something that system
daemons never do. For system-wide, this is in
/etc/X11/Xsession.d or whatever your system uses.