<p>I want to install a gem on all my application servers, but gem install requires sudo access - how can I enable sudo only for running this capistrano command? </p>

<p>In other words, I don't wish to use sudo for all my deployment recipes, just when I invoke this command on the command line.</p>

I'm not quite sure I understand the question, but I think you're asking how to restrict sudo to the one specific command and not have to grant unlimited capacity for mischief to all of your Ruby developers.
/etc/sudoers can be set up to restrict the commands which users are allowed to invoke as root. It is commonly set to ALL, but you can provide just a list of the allowed commands.