If you have access to screencasts through Prime (which I think all forum-enabled plans do), Drew Neil, the author of Vimcasts, has a screencast here on Learn where he describes how to include tags for gems and/or the standard library in your tags. I’m going to sum up that video quicky here because I just caffeinated and have nothing better to do this early in the morning.
I’m using the approach, and it’s pretty awesomesauce. If you’re an rbenv user (and you really should be, if I’m going to stand on my soapbox for a moment), you can index the standard library whenever you install a new version of ruby with vim hero/celebrity/benevolent dictator/philosopher king Tim Pope’s rbenv-ctags. Then I index gems using gem-ctags, also written by the aforementioned Tim Pope.
I think that solves your problem, but I’d also recommend having ctags get automatically built whenever you git commit using a solution devised by some guy named Tim Pope (seeing a pattern here?) that he documents in his blog: http://tbaggery.com/2011/08/08/effortless-ctags-with-git.html
Hope this helps!