title: Are all gems used required in the Gemfile?
body: <p>I'm pretty new to Ruby and have been reading up about Gemfiles and the like.  Recently we were bitten by this bug in one of our programs:</p>
  
  <p><a href="https://github.com/ffi/ffi/issues/440" rel="nofollow">https://github.com/ffi/ffi/issues/440</a></p>
  
  <p>However, in looking through the Gemfile in question, the <code>ffi</code> gem wasn't listed.  It is now, pinned to a prior (working) version for the OS in question.</p>
  
  <p>But I'm wondering how things worked before if it wasn't in the Gemfile to start?  Are there some 'core' gems out there that come along for the ride, with no need to put them in a Gemfile?</p>