(1) You have to SBF down to System Version 2.4.330 or 2.4.229 (which are Froyo 2.2 based) as those system versions contain radios that accept the TBH Band Unlock Hack
(2) You must obtain a SIM unlock code for your device from Verizon (or another 3rd party vendor)
(3) You must root, install Bootstrap Recovery, and then while in recovery, install the TeamBlackhat ZIP file that hacks the radio.
HOWEVER...I took a weird path in this, and was still successful.
When I first got my D2G I SBF'd it to the Gingerbread .608 system, and then set everything up the way I wanted it to. (rooted, installed apps, configured accounts, etc). I then made a Nandroid backup in Recovery.
I then SBF'd down to 2.4.330. I applied the TBH Radio Hack and then did the SIM unlock code. I now had a happy system running 2.2 and functioning on AT&T's network!
But of course, I needed to get back to Gingerbread. An additional hack was made (that can be found on Rootzwiki) to put yourself onto the newest D2G stock firmware without touching the radio and ruining the Tmo/AT&T Hack.
So I went back to recovery, restored my Nandroid of .608, and then flashed the .608 kernel. I now have my .608 system completely intact and still riding on AT&T.
It's hard, it's a bit complicated, but once you get your version numbers straight you can flash at will.