edit .xinitrc in ur home dir
put all the initialization stuff before u start Gnome
Here is a sample of my .xinitrc
#!/bin/sh
xset +fp $HOME/.fonts
chbg -scenario $HOME/bin/sc1 &
exec /usr/X11R6/bin/fluxbox
Now in ur case replace fluxbox by gnome-session or whatever appropriate on ur sys
remember to put '&' sign for ur script
HTH