(Covers information from the D.C. comics)
According to Barry Burton, the G-Virus is an updated form of the t-Virus. Its main difference from "t" is that it can be spread between people, whereas Zombies infected with the older virus cannot.
The somewhat contradictory descriptions of the G-Virus is due to the indepenence of storywriters. One writer portrayed it as an entirely unique virus, while Ted Adams ("Dangerous Secrets") portrayed it as an enhanced t-Virus with the same effects like Zombification.