There are places in the internet, where you can get a free digital signature for email.
How can I use it with Gmail and Internet Explorer?
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I you use Firefox, you can use Firegpg ( http://getfiregpg.org)
From the comments in this Gmail support forum thread, it looks like you may either have to use a desktop mail client (such as Outlook or Thunderbird), or use Firefox instead with extensions to allow certificates to be added to Gmail.