Here's a screencast in case you want to watch instead of read.
I was curious
what versions of Rails were remotely available to be installed with gem. On
its own, gem list rails will show all locally installed gems that
partially match rails. That'll include gems like rspec-rails and
sprockets-rails.
First, the --exact flag can be added to narrow down the results to an exact
match of rails.
Then, the --remote flag can be included to request gem look for remote
versions. That is, versions available on the rubygems server.
Lastly, the --all flag can be included to fetch all versions instead of only
the latest version.
Putting it all together:
$ gem list rails --exact --remote --all
rails (6.1.0, 6.0.3.4, 6.0.3.3, ... 0.9.0, 0.8.5, 0.8.0)