Hi,
It's GPLv3+, as all sources files have headers that say:
* either GPLv2 or any later version
* or GPLv3 or any later version
Moreover upstream website says GPLv3
http://sourceforge.net/projects/tikzit/
But the COPYING file provided is GPLv2, looks like a mistake,
ask upstream to fix this in their source control.
Regards,
H.