Do I have to manually uninstall all dependent gems?
Solution 1:
gem list | cut -d" " -f1 | xargs gem uninstall -aIx deletes all installed ruby gems!
Solution 2:
As far as I know you're correct, there is not an easy way built-in to the gem command to do this.
However, you can check out gem-prune which can help clean up your gem repository after you've removed dm-core.
http://github.com/ddollar/gem-prune/tree/master