Since this is for a Dreamcast emulator, that method is not an option. Although a standard CD-ROM can recognise a GD-ROM, it can not read the GD data track. CDRwin/FireBurner have no methods for ripping this data either.
The only current methods of ripping a GD-ROM to an image are:
1) Attach your DC to your PC either through a network (DC will need a BroadBand Adapter) or the DC-PC serial cable (will be very slow). You then code a program that can tell the Dreamcast to read the GD-ROM and send the data along the cable and the PC then stores it.
2) Get a GD-ROM drive (assuming they were ever made for PCs, and if they were available to the public). You wouldn't need to ISO then.
Of course, this is all relatively pointless since no DC emu is at a stage where playing commercial games is worthwhile. Cyrus64 stated that if his Icarus emulator ever gets to this stage, he will implement a method similar to 1, except it will just read the necessary data as the game wants it. This will require an actual DC and the BBA for speed reasons.