Gemspec: How can I specify dependencies which don't have to be auto-required?

Gems specified in an engines gemspec file already do need to be explicitly required, by default. From the official documentation - Note that if you want to immediately require dependencies when the engine is required, you should require them before the engine's initialization. In your case, you should be able to get by with something like gem.add_dependency 'uuidtools', '2.1.3' in your gemspec file, and requires in the relevant locations.