$ sudo apt-get update
$ sudo apt-get install libgnutls26 

libgnutls26 is installed in your system.

Make ensure the libgnutls26 package were installed using the commands given below,

$ sudo dpkg-query -l | grep libgnutls26 *

You will get with libgnutls26 package name, version, architecture and description in a table

libgnutls26 package basic information: