After some trial/error I figured out that babun doesn’t support ruby/gem installed with pact.. So I to fix this I did
Remove the ruby pact packages

pact remove ruby
pact remove rubygems

The next commands must be done in the normal windows Command Prompt!
Download the Ruby [Windows-based installer](http://rubyinstaller.org/) (which installs also rubygems), during install choose to add it to PATH, so wehn you run

ruby -v
ruby 2.2.3p173 (2015-08-18 revision 51636) [x64-mingw32]

It shows the version number
Add an alias to the new installed gem

alias gem='D:/tools/ruby/bin/gem'

Now if you run

gem -v
2.4.5.1

You get the version outputed.
Last step is to install the [Ruby Development-Kit](https://github.com/oneclick/rubyinstaller/wiki/Development-Kit)
Enjoy 🙂