python2.5-gtk2 has already been uploaded to diablo-extras, so it should be available when you do an "apt-get install python2.5-gtk2" on a scratchbox terminal...

From the repository it looks like a lot of python2.5-related packages are already uploaded:

http://repository.maemo.org/extras/pool/chinook/free/p/

Did you configure properly your /etc/apt/sources.list in scratchbox?