You can download a clone of the software here:http://www.c128.net/infos/gb128info.htm
The "hardware" part really only was the update to 64K RAM. Apparantly Combo used a specialized way to do this: Since the VDC-RAM isn't socketed mostly, but the VDC chip is, you had to remove the VDC chip, put the board in its place and put your VDC chip on the board.
Later when the 128DCR came out Combo sold a software-only version, since the computer already had the 64K VDC.
The 256 or 65000 colors are pretty close to a lie I think. It's possible to get some strange colors on the Commodore 1901 Monitor if you shift the picture to the right (or using the VDC registes to do this I think). But I haven't seen any useful implementation of this. And from what I know about the Graphic Booster it only does this by dithering and interlace. Certainly NO special hardware that only Graphic Booster had.