From that log it looks like the Ruby installer did not add Ruby's directory to your PATH environment variable. Perhaps it was already installed before with the "add to PATH" option disabled? (
http://rubyinstaller.org/downloads/)
reapack-index's installer should pick it up if you manually add the directory containing Ruby's executables to your PATH variable. Usually it's "C:\Ruby23\bin".
Another solution is to install it manually with the "gem install reapack-index" command, but on Windows it's a bit cumbersome as the dependency to rugged must be built from source (if you go down this route, see
here for pre-built 32 and 64-bit versions).
(You didn't specify whether you wanted to create your own repository, but just in case you didn't already know: installing reapack-index is not necessary for contributing to the public ReaTeam repositories.)