ruby-prof has been around forever. Combine it with kcachegrind and you have a very powerful toolset. I wrote about instrumenting Rails apps here: https://www.coffeepowered.net/2013/08/02/ruby-prof-for-rails...

The same concept applies to gems. You just construct a scenario that stresses your gem internals, run it with ruby-prof, pop open the resulting dump in kcachegrind or similar, and explore to find the hotspots. I did something like this for MongoMapper, and documented the process here: https://www.coffeepowered.net/2013/07/29/mongomapper-perform...