Now you can add your location to your Gmail signature. There's a new Lab feature introduced yesterday that detects your location and appends the city region and country names to your signature.
To enable the Signature feature simply head over to your Lab settings in Gmail and turn on the Location in Signature feature. Then go to your signature preferences and check the box next to "Append your location to the signature."
But the location in your signature might be a tad less accurate as this feature uses the public IP of your computer to determine your location, so it might show Germany if you are at the Heathrow airport. To make your signature a tad more accurate, make sure your browser has a version of Gears that supports the location module. That way, Gears can make use of wi-fi access point signals to recognize that you're actually in London.
And then again, you would always have the option to either disable the option or just delete your location from specific emails whenever you want.