The last Ruby Weekly News has something that might be helpful here:

http://rubygarden.org/ruby?RubyNews/2005-01-03

Tanaka Akira posted a patch to record GC-related information:

* the total number of GC invocation
* the number of GC invocation, when an object is collected
* the location where the last GC is yielded
This patch might help you if you are interested in GC internals.