Just got the registration verification e-mail. Bought X-COM with the coupon code for $16. Everything went through just fine. I paid with PayPal and both my GMG and PayPal e-mail address were identical.
Regarding what qhplar said, it does say somewhere on the billing information page that your PayPal and GMG e-mails need to be the same. I can't confirm or deny what he says, but it should work if they're the same.
Yup, the email address registered at GMG and Paypal have to be the same or it doesn't work. I have 2 GMG accounts since it's the only way to buy 2 copies of the same game and I have to use a credit card for one & Paypal for the other. I've never had any issues with GMG and their sale prices are great (been buying from them for about 3 years now).