Yeah, the coupon is legit; its just a screenshot of the email you get when you subscribe to the Gamestop email newsletter. As previously mentioned, just subscribe once and hang on to the email. The html in the email displays the images from gamestop's webserver. They simply update the images to reflect a newer expiration date, so when you look at the original email again it will magically expire a month later.
That is, until they catch on that all us CAGs are using the coupon over and over and decide to do it a different way. I'd imagine that won't happen for a while though.
The cool thing is, you can't really get in trouble for using it again with the new expiration date, because its still just sitting in your inbox. Its not like you even have to sign up with multiple addresses or anything.