Look at this, doesn't it look secure?
Personally, whenever I see a mention of signing or encrypting a mail message I'm like, whatever... Whenever I get an e-mail with a message digest to proof it hasn't been changed by some evil third party I am like, pff, arrogant prick, who do you think you are assuming somebody even cares enough about you to change your e-mails? Do you honestly think I'm going to actually check this message hash or if this is done automatically that I'm going to take your mail more seriously? I don't think so.
And honestly, if you would send business critical PGP encrypted contracts and stuff, would you really send it from Gmail? Come on.
Thankfully I hardly ever receive these "signed" e-mail messages, and if I do it's usually from some nerd security fanatic, the kind that puts public keys on their business card to pick up girls (good luck with that).