"G" refers to a type of irregular mutant that appears in the Resident Evil video games. These creatures are products of the G-virus, a highly unstable mutagen created by Umbrella scientist William Birkin.
The G-virus was first discovered by Dr. William Birkin during experimentation on Lisa Trevor in Umbrella's Arklay Research Laboratory. It was first produced as a by-product within Lisa's body, which had been exposed to multiple viral strains during her incarceration. As it turned out, the G strain possessed fierce regenerative qualities and would allow those infected to rapidly heal and grow stronger, essentially granting its hosts biological immortality.
Birkin conducted extensive research on the G-virus at Umbrella's underground lab complex beneath Raccoon City and completed his work in September 1998. He had originally intended to use the virus to enhance his position with Umbrella, but he began to disagree with the motives of his superiors and decided to keep the research to himself. He attempted to contact the US military so that they could retrieve him and pass on his research to the government, but Umbrella intercepted Birkin before he could be retrieved and attempted to seize a virus sample. Birkin was gunned down by Umbrella troops who took an attache case containing several vials of the T and G viruses. Birkin managed to save himself by holding on to one vial of G, which he injected himself with and underwent a rapid and horrifying transformation.
Upon his transformation, Birkin's powers of thought and reason quickly deteriorated, and he became driven by the virus itself and sought to infect other potential hosts. He produced parasitic embryos within his mutating body and attempted to implant them in others.
G embryos appear as hand-sized pink creatures that look somewhat like tadpoles. They are spawned by G-adults and are normally implanted via a victim's mouth. They cannot survive long on their own, but they will grow rapidly within a host, reaching a suitable level of maturation in mere minutes. If the victim is of a blood type similar enough to the implanter, it will be doomed to become an adult G similar to the original. This is believed to only occur with subjects that share genetic bonds (family members; parent and offspring or siblings). Incompatible hosts are killed by their parasites, which eat their way out of their hosts and then quickly grow into adulthood.
G adults (incompatible)
Adult G creatures born from unsuitable hosts will grow to adulthood rapidly after discarding their hosts, taking a grotesque shape with abnormal features. The final shape they take is not always the same with each subject, but the mutation stops at a certain point and does not continuously alter itself unlike first-generation G's. Incompatible G adults also do not possess the same level of regenerative ability that first-generation subjects do. They do, however, all share one feature found on all G-types: the appearance of a large eye forming on a random part of the body. While not on the same level as first-generation G's, these creatures are still extremely dangerous and possess great brute strength as well as the ability to spit poisonous fluids at prey and spawn new G embryos. All incompatible G adults encountered so far have developed excess body mass that hindered their movements, making them easy to evade so long as they are not fought in enclosed spaces.
First-generation G adults
First-generation G's are subjects that have either been directly injected with pure G-virus or share blood-ties with someone who has been injected. These G subjects gain an incredible degree of regenerative ability and will mutate constantly, their bodies changing shape regularly (time between transformations varies with different subjects and their circumstances). Only two first-generation G subjects are known to have existed: William Birkin - creator of the G-virus - and Curtis Miller, an anti-bioterror activist.