Open a command prompt as admin (click start button, type cmd in search bar, right-click on cmd.exe in the list, click on "Run as administrator"), copy/paste this and hit enter:

c:\ruby200\bin\gem.bat install gtk2 --source http://rubygems.org --no-ri --no-rdoc

then copy/paste the results here. The command prompt kind of makes it seem like you can't copy from it. Just right-click, click select all, hit enter, come here and paste it.