$ sudo apt-get update
$ sudo apt-get install ruby-fftw3 

ruby-fftw3 is installed in your system.

Make ensure the ruby-fftw3 package were installed using the commands given below,

$ sudo dpkg-query -l | grep ruby-fftw3 *

You will get with ruby-fftw3 package name, version, architecture and description in a table

ruby-fftw3 package basic information: