This is at the first results in google but here is a shortcut:
gem list | cut -d" " -f1 | xargs gem uninstall -aIx
This will remove all gems if you are developing in Linux. Here is more detailed description.
For windows developers there is also a quick tip. You have to start PowerShell to run that (hit start button and just type it, if you are using Windows7)
gem list | %{$_.split(' ')[0]} | %{gem uninstall -Iax $_ }
And the detailed explanations if you want to learn the commands.