Make a Gemfile as usual, and place it in any folder. It does not need to be a project folder. Then you can just do:
bundle install --system
This will install the gems in the Gemfile system-wide, and will ask for the root password if you do not have access to the system folder.
--system: Installs the gems in the bundle to the system location. This overrides any previous remembered use of --path.
You can also name your Gemfile(s), if you want to organize them in some way:
bundle install --system --gemfile=/path/to/my_special_gemfile