Gems are installed by sources, or a repository which holds several gems. Generally all the gems you will need are either in gems.rubyforge.org or gems.github.com, so once you setup these two sources you don’t need to setup more sources until you need a gem from somewhere else.
How can I see the sources my gem is using?
gem sources
How can I add a source?
gem sources –add http://gems.github.com
How can I see the gems which are installed on my system?
gem list
What if I want to search for gems by name?
gem list -r | grep somegem
(where “somegem” is the name of the thing you’re looking for)
How can I see some information about my gem environment?
gem environment
How do I install all the gems needed for the app I’m working on?
rake gems:install