when I run gem install rmagick-2.13.1.gem from the directory in which rmagick-2.13.1.gem is I get an Error saying that it failed to build gem native extension, belo
gem install rmagick-2.13.1.gem
Download zip file from: https://github.com/rmagick/rmagick/downloads
Unzip it in a local directory
CD to that directory and type
gem install rmagick --local
Done.
If you get errors you might want to read the Readme.html file in the Zipfile