I'm not sure I understand what you mean... anyway, I hope I do
So, if you want to start gnome when you type startx on your prompt, try to edit your ~/.xinitrc by writing there on the first line gnome-session. This will start gnome when you're trying to start the graphical interface (when you type startx).
In case that you want to start graphical automaticly when you start your computer, you should edit your /etc/tyys file and change the line that starts with ttyv8. On my computer I use KDE and it looks like this:
ttyv8 "/usr/local/bin/kdm" xterm on secure
On your computer, I gues that you want to start gdm... then replace "/usr/local/bin/kdm" with the absolute location of your gdm file. You can find it by typing "whereis gdm"
I hope this helped...
Bye !