When I include a gem that I made, thanks to Bundler (version 1.0.12), in a Gemfile and then I try to bundle or to rake just like that:
$ rake
I\
As Ben Hall said, you must fix the gemspec file that may change for system to system. To know what file is see what file is tell in the error report, for example:
Invalid gemspec in [/var/lib/gems/1.8/specifications/svn2git-2.1.2.gemspec]: invalid date format in specification: "2011-12-28 00:00:00.000000000Z"
In this example you must edit "/var/lib/gems/1.8/specifications/svn2git-2.1.2.gemspec" file and change "2011-12-28 00:00:00.000000000Z" for "2011-12-28" in s.date option.