If you want to get version information about a gem the easiest way to do it is with this command
gem listFor example
gem list activemerchantwill output the activemerchant versions I have installed. You can pass only part of the name like
gem list aget the names of all gems (and versions) that start with "a". There are a lot more options so here is a link to the gem command reference http://rubygems.org/read/chapter/10
The commands here are useful if you've installed a gem but the version you've configured in your environment.rb file is different. To quickly grab the right version, this command is pretty handy.