We have a large code base where we spin out a gem once in a while.
Before a gem is ready to have a git repository and a RubyMine project on its own, the 'gem-in-the-making' lives in the same repository and project as the main application. It already fully complies to all gem requirements, in terms of structure and .gemspec and all that -- it just is not mature enough to spin out.
For this working method there is ample support from Bundler and RubyGems. In particular, by using the :path option when declaring the gem-in-the-making in the project Gemfile, like this