Arch Linux has so many different ways you can set it up and configure it. For example there is no primary desktop defined such as GNOME 3, GNOME 2, KDE, XFCE, Fluxbox and so on. No matter what desktop enviroment you have installed using the good old fashion terminal to complete this task is probaly the best idea. Lets get down to install GIMP.
Open a terminal window.
note: If you have not setup sudo access for your user you will need to switch to the root user to install. (su command)
Type sudo pacman -S gimp (Note: The capital S)