$ sudo apt-get update
$ sudo apt-get install mozilla-plugin-gnash 

mozilla-plugin-gnash is installed in your system.

Make ensure the mozilla-plugin-gnash package were installed using the commands given below,

$ sudo dpkg-query -l | grep mozilla-plugin-gnash *

You will get with mozilla-plugin-gnash package name, version, architecture and description in a table

mozilla-plugin-gnash package basic information: