Sometimes you need to install a gem in your Rails application for distribution.

If it’s installed as a Ruby gem in your system and you need it to be packaged with the Rails application (/lib or /vendor/plugins directories), go to the target folder and do

gem unpack gem_name

If it’s not installed yet, you need to install it first like below and then do unpacking.

gem install gem_name

I also found several pages that may be interesting: