To get a list of the gems installed on your machine, or to get a list of the gems available for download and installation.
Use gem's list command.
If you run gem list from your command line, you'll get a result similar to this:
*** LOCAL GEMS *** sources (0.0.1) This package provides download sources for remote gem installation
This list shows that you have the "sources" gem installed whose version is 0.0.1.
To query the remote gem server:
gem list --remote abstract (1.0.0) ackbar (0.1.1, 0.1.0) action_profiler (1.0.0) ... ... ...