the tk extension extracted from core and gem-ified gem install tk_as_gem
originally extracted from 1.9.1p376, then added a minor change to make it work with mingw/msys.
To install into an existing mingw distro: install ActiveTcl [c:tcl] (should add itself to the path) C:>gem install tk_as_gem
Note: ruby.exe and your tcl dir must be on the same drive.
Might install for linux, as well.
Note that this only works for 1.9 – for 1.8 mingw you can install some binaries from here: github.com/rdp/ruby_windows_tk
currently to exit out of ruby, once you've done a require 'tk' of the gem, you must first close all outstanding open Tk windows, THEN exit your ruby process, otherwise it will hang.
Also you “might” need Tcl/Tk 8.4 instead of 8.5 (you certaintly do, for 1.8)