ruby-1.9.2-p290@demo ~: gem update
make install/opt/local/bin/gmkdir -p /Users/crusso/.rvm/gems/ruby-1.9.2-p290@demo/gems/json-1.6.1/ext/json/ext/json/extmake: /opt/local/bin/gmkdir: No such file or directorymake: *** [/Users/crusso/.rvm/gems/ruby-1.9.2-p290@demo/gems/json-1.6.1/ext/json/ext/json/ext] Error 1
I had no gmkdir in /opt/local/bin... no gmkdir at all. Previously, I had been using Macports, so I guess that ruby gems had cached the location to gmkdir.
Rather than blowing away my whole rvm installation and starting over, I installed the coreutils module with brew:
brew install coreutils
That includes gmkdir, but not in the right spot. So, I created a symbolic link to it:
sudo ln -s /usr/local/bin/gmkdir /opt/local/bin/gmkdir
Let me know if this problem was plaguing you and this helped.