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The gems dialog should have a cleanup pane listing the older versions of gems (output of gem cleanup -d) and a way to remove particular versions of each gem.
Great idea. Rather than another pane, I would add "Clean up..." button in addition to "Uninstall" button in the "Installed" tab. It will popup some list of gems which would be uninstalled if subsequently confirmed by the user. Likely not into 6.1.