If you want to unpack gem file - you can only unpack gems that you have installed, for example memcache-client:
gem unpack memcache-client
You may also use rake:
rake gems:unpack GEM=memcache-client
But some reason, the second does not work for my msysgit.
And, there is a discussion to use gem or plugin (http://www.robbyonrails.com/articles/2009/01/13/question-plugins-or-gems-or-both)