Error when installing devise- i am a novice

I have installed rails and am running the Git Bash and am on windows 7 64 bit, i have installed gemfile and updated and this is my first use of ruby! I am following the instrustions from devise's github http://ift.tt/LkuK7q . I am probably being very stupid but when i run gem 'devise' and this error occurs:

ERROR: while executing gem... <Gem::commandlinerror> unknown command /devise

Any help would be greatly appreciated