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

Yet here it is installed:

gem which P4
(checking gem p4ruby-1.0.7 for P4)

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