I had many problems trying to get my d2g rooted and get a custom rom installed. However, the biggest problem was that I had a hard time finding good information. A lot of what is here is either outdated or does not apply to the d2g.
As others in this thread have suggested the z4root app does work on a fully updated stock d2g. After getting root through z4root, you should download ROM Manager from the Market as well as d2/dx bootstrapper. Flash ClockworkMOD Recovery from ROM Manager, and then use the bootstrapper to bootstrap your recovery. Search these forums for a download of Fission 2.4.3 (please note that's 2.4.3, not 2.5.3 as previously mentioned in this thread) and copy the .zip to your SD card. I had trouble getting ROM Manager to flash it, so I would suggest booting your phone into recovery (easiest way is to tap "reboot recovery" from the bootstrapper app) and then flash Fission 2.4.3 manually from there. Any other d2 rom besides Fission 2.4.3 will lock your phone in airplane mode, screw up your keyboard, and lots of other neat stuff.
Hope this helps.