Yes, you will need X11 to run Gimp on your Mac. You can find the X11 package you need on your Tiger install discs (you can't use the one that is downloadable from Apple so don't bother).
To make Gimp a little more "intuitive", you can also download and install GimpShop. It adds an interface to Gimp that is more like Photoshop and much easier to navigate.
Just insert the discs that came with your computer
Look for 'Optional Installs' and select it
In the installer you will find 'X11' - put a check next to it and follow the instructions from there
It's pretty easy, have to have it for Open Office also
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