How to get apps to start up on GDM startup?

[Travis Newman]

I have a howto to get the thumb buttons working here:
http://www.ubuntuforums.com/showthread.php?t=3828
but the problem is that it only works for startx users, because the 
xmodmap command won't work with the sessions applet's startup section 
for some reason. Is there a file like .xinitrc for GDM, or is there a 
way to edit the specific session file (ie. Gnome and KDE entries for the 
GDM sessions) to start it up? Thanks!

Travis