apt-get does not install dependencies
Solution 1:
Install aptitude using:
sudo apt-get install aptitude
Then do:
sudo aptitude install libgnutls26
Aptitude will show the problems and propose a solution.
The first proposed solution will be to keep everything as-is. Answer 'n' to that question.
The next proposed solution will be to downgrade all of the dependencies that are currently too new. Answer Y to that.
Then, try installing libgnutls26 again.
Solution 2:
Not sure if there is conflicting dependencies already installed, but go to the terminal and type sudo apt-get update
and then sudo apt-get upgrade
Tell me what message (if any you get like a 'warning...' or 'error..') you get. Also if you are trying to install GnuTLS go to synaptic package manager in your system tools menu, and do a search for it. When you find it right click and select install, and it will bring up the dependencies you need and install them as well.