A big advantage of Gemnasium is its push nature, rather than pull. You could use bundlers outdated command as well, but when you have multiple projects keeping up with that becomes burdensome.
Don't get me wrong. It seems wonderful for those who have many projects. If you start supporting things other than gems (sbt for scala, maven for java, lein for clojure, etc), it can be a valualbe tool for many enterprises. Specially if you highlight which updates have security updates.
Nevertheless, at the moment, gemfresh is enough for my personal projects.