Oops yes, I forgot to advise you to edit your /etc/apt/sources.list file.
Because Debian releases have moved on since Knoppix 5.1.1 came out, the meanings of stable testing unstable and so on have shifted.
I find that Knoppix 5.1.1 successfully installs or upgrades packages from what is now the 'stable' (designated 'etch') repository.
So it's safest to comment-out all URL statements for testing, unstable and experimental.
I also like to edit 'stable' to 'etch' , which is clearer and more future-proof for when Debian goes up to the next full release number.
Then do apt-get update again.
Ensure that there are no 'aborted' configuration files on the system by
dpkg --purge libgtk2.0-dev
and re-try
apt-get install libgtk2.0-dev
Good luck