I use a ATT Uverse gateway 3800HGV and it was a huge pain in the @#%! to figure this out, portforwarding.com and other sites and many people just couldn't tell me exactly what to do.
Well I finally figured it out late last night, if anyone else has one of these pesky broadband router/gateways and is curious how to do this (While also running Win7 64bit) PM me and I will tell you how I did it
The secret lied in a new software update that allowed you to assign a FIXED private IP#.
Although I have not yet hosted a gettysburg game, I hosted several warcraft 3 games last night and I have never been able to do that succesfully.