Where are my ruby gems?
Running gem environment should give you a fair bit of information about your RubyGems installation. The field you're looking for is 'INSTALLATION DIRECTORY'.
That said, if you want to modify some gems, there may be better ways to go about it. Most gems, for example, are hosted on github.com. If you make a fork there and make any improvements, you can submit them back to the maintainer in the form of pull requests. Most developers are happy to accept patches (that's why they're on github, right?), and you get a warm fuzzy for making the world a slightly better place for everybody else using that gem.