Okay - there was never anything officially called a G1. Here is the layout of the different generations.....
circa 2000 - the Taurus Millenium was released. This is what you referred to as the "G1" as it was the first generation, and pre-dated the Mil Pro. Double action only with heavy long trigger pull.
The Millenium Pro generation 2 was released a few years later, the gun was very different to the careful observer, and the mags changed as well. Double action only with heavy long trigger pull.
The Millenium Pro generation 3 was released a couple of years after that, ushering in the second strike capability, and the action changed to SA/DA with a pretty light trigger pull. Mags are the same as the second gen pistols. Sights went from screw-down to dovetail.
G2 is a reboot of the previous Millenium and Millenium Pro guns, and the 24/7 line. They primarily have a DA/SA trigger pull, the first round being double action and subsequent shots being single action. The magazine changed again with this release.