well yeah you need ruby to run a ruby gem?
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.
If YOU read the content that was linked, you'd see there is a very long README targeted at people who aren't Ruby developers.