It is using mruby at the moment, but all the mruby gems are included (which isn't a massive amount, to be clear). We're going to wire up a more standard require(), but for now $dragon.require('app/myfile.rb') works.
We don't provide access to C (but our intention is to try to supply many reasonable things you would want, callable from Ruby; we take requests!), but for pure Ruby gems, it's possible they could work.