possible to set gems path?

I've installed ruby and rubygems on leopard. when i install a gem successfully and run require "gemname" in irb it fails:

irb(main):001:0> require "P4" LoadError: no such file to load -- P4   from (irb):1:in `require'   from (irb):1 irb(main):002:0>

Yet here it is installed:

gem which P4

(checking gem p4ruby-1.0.7 for P4) /usr/local/lib/ruby/gems/1.8/gems/p4ruby-1.0.7/lib/P4.rb

Should I set an env var? If so, which and where?

-Johnny