If you take a look at gitk code, you will find that everything occurs inside threads (more or less). That happen to discourage the use of debuggers, which are fooled by this flow, hanging or detecting infinite loops

too many nested evolutions (infinite loop?)

To debug gitk, this is my Q&D: I found you should comment the line

tkwait visibility .

put a breakpoint on

wm ...

And step through a few lines (seems like something runs out of the debugger control that way). Then continuing the execution doesn't hang, and any other breakpoint you set will work.

It would be worth to add an option to run gitk in --debug mode #2