I had ruby - I was missing the ruby-dev header files.
My original complaint though is that people are using "gem install" as the sole documentation for installing a general purpose command-line tool. I'd rather not even know it's written in Ruby (I just want to install it and use it) - instead, I need deep Ruby knowledge just to get the thing running.
It's not generally obvious to non-developer end users that they will (sometimes) need to install the -dev package on their OS. Would be nice if it were. I'm mostly taking issue with the unfriendliness of the comment.