Here's a quote from the unofficial FAQ
http://www.misplaced.net/fom/X11/39.html
The .xinitrc file needs to go in the base of your home directory. Use your favorite text editor to create and edit this file, a simple .xinitrc is as follows:
---------------------8<-----------------------------
# This line sets your path environment
PATH=$PATH:/usr/X11R6/bin:/sw/bin:/usr/local/bin
export PATH
# Opens up an xterm
xterm &
# opens up the oclock clock
oclock &
# opens up the fun xeyes program
xeyes &
# Runs the quartz-wm so you can interact with the applications you've started
/usr/X11R6/bin/quartz-wm
----------------------8<----------------------------
You need to make sure that a window manager is listed or you will have applications open that have no title bars that you can't move or minimize.
So what this text file does is tell X11 what to do when you start it. The # signs denote comments until you hit return for the next line. The 3 lines in the middle that say 'xterm &', 'oclock &', and 'xeyes &' are all examples of programs that will automaticallys start up.
You can change the 'xterm &' to 'gimp &' and put # signs down there in front of the oclock and xeyes (or just delete those two and their comments).
Now when you start X11, gimp will be what is launched.
I hope this works out for you!