Linux?

The executable is /usr/bin/wish. On my (RH) system it comes from the 'tk' package.

You can do without this. Just look in the directory where guiscript lives. You can use the command-line tool gui_test:

./gui_test -show_graphics window
./gui_test -show_graphics fullscreen

You need to do this when boinc and einstein are both running.

Note that within a few weeks a BOINC graphical core client for linux will be available, just like for Win32. I have it on my laptop for testing, but it's not ready for widespread use yet.