By default, the Ruby GC doesn't give a profiler much info. However, one can patch the Ruby GC to expose more information that can be used by your profiler. Once the GC is patched, you can use a Ruby profiler like ruby-prof to see how much time is spent in your code and how much time is spent in the GC.
Here is a great article that explains exactly how to set that up:
If hacking Ruby sounds daunting to you and you're looking for a quick and more user-friendly way to monitor the performance of an entire rails application, you should check out New Relic RPM. New Relic is currently the defacto performance analytics tool for Rails developers, and has a free "lite" version of their service that provides some pretty useful functionality.