FIXING THE GITK Mountain Lion DISPLAY PROBLEM
As I, many others have noticed after installed or upgraded to OS X 10.8 Mountain Lion, that gitk wasn't working properly as it requests a missing DISPLAY variable.
Occurs that, Apple is no longer including its X11 application for running software through the X Window System interface. Gitk was totally based on it.
Now a popular option to run specialized UNIX software over the Mac side is XQuartz project. Go to http://xquartz.macosforge.org/landing/, and dowload the latest version.
When you install Xquartz, log out, and log in again, it will, forever after, set the $DISPLAY variable automatically for you.
It should look something like the following:
$ print $DISPLAY
And gitk should start work all over again.