extrated from Yehuda's:

```
If you find that you need to develop against a gem that hasn’t yet been released (for instance, Rails often develops against unreleased Rack, Arel, or Mail gems), you can add individual lines in the Gemfile that tell bundler where to find the gems. It will still use the dependencies listed in the .gemspec for dependency resolution, but it now knows exactly where to find the dependency during gem development.

source "http://www.rubygems.org"
 
gemspec
 
# if the .gemspec in this git repo doesn't match the version required by this
# gem's .gemspec, bundler will print an error
gem "rack", :git => "git://github.com/rack/rack.git"

You wouldn’t want to include this information in the .gemspec, which will eventually be released to Rubygems after the in-development gem has also been released. Again, this makes the overall system more resilient, as it doesn’t depend on transient external URLs. This information is purely something used to set up a complete environment during development, which requires some precision.```

I understand that git gems goes on gemfile, not on gemspec; anyway, I've tried to put it on gemspec instead of gemfile... and it doesn't work.