I have a script that I need to start up after the user logs in via a nx client. From what I understand, Gnome starts up after a user logs in (correct me if I am wrong here).
GNOME Ships an Application called "GNOME Tweak Tool" or simply called "Tweaks". This does have an section called "Startup Applications".
After you added something to autostart, i recommend to you too look in ~/.config/autostart what this tool did.