The easiest way to send encrypted email with Gmail that I have found is to do so with Firefox and the FireGPG plugin. You can either encrypt the message before typing it into Gmail or you can type the message and then highlight the text and run the FireGPG plugin to encrypt it. Below are a few simple steps that should help encrypt email you are sending through Gmail to someone who has a PGP key.
NOTE:The below steps are if the FireGPG buttons are not showing in Gmail and is a temporary fix until FireGPG is updated and the entire message can be encrypted. I will update this with a link to the new article/howto once FireGPG is updated.