Sometimes you need to view the full email headers to understand whether an email sender information or other details is faked or to even debug your MTA (my situation). Unlike other email clients GMail doesn't give you an intuitive way to get the full headers. Nevertheless when you know it, it is very simple. Here are the steps:

1. Open the email in Gmail
2. Click on the combo-box titles Reply and select Show original. That's it!

The email with full header details is displayed in a separate window / tab (depending on your browser configuration).