There currently does not appear to be any really good torrent file parsers available via the gems system.
rubytorrent seems good at first glance but requires some maintenance to handle udp trackers, which has been done by dydx available on github. As I cloned the repo rather than install it via gems the requires do not work properly so I have cloned it and modified the way it requires files.
Other torrent parsers available are aversa and Bencoding. Bencoding was originally found here but I have modified it a little bit to make it more user friendly here and an example of how to use it here.
A quick Review of Performance (parsing a handful of torrent files)
Bencoding Execution time 96.626849 seconds
Aversa Execution time 10.573079 seconds
RubyTorrent Execution time 0.689869 seconds