The code isn't game specific.
I just bought Guild Wars 2 Standard Digital Download Pre-Purchase for $47.99. I got my Pre-purchase code immediately. It should work for the DDE too.
I tried to get in on the GMG 25% pre-purchase earlier, but they wouldn't take my money and didn't answer my support tickets. Since GMG is now out of pre-purchase keys, this is a better deal at the same 20% off price with no hassle and no foreign transaction fees.
EDIT (thanks to theflicker):
Here are the limitations:
This coupon may only be applied to digital game, pre-order digital game, downloadable content, pre-order downloadable content, or bundled items.
Edited by mikon, 14 August 2012 - 08:08 PM.