The obnoxious and unpleasant son of Ronald Rust and heir to the Rust land and title. He is first encountered as the Rust son who was arrested by Sam Vimes and brought before the Patrician for shooting a servant for allegedly setting his boots out the wrong way round.
In Feet of Clay he (or his father) was later the chief suspect in paying for an assassin to kill Vimes as revenge. The assassin failed, discovering too late the lengths Vimes had gone to make the Ramkin house a lethal death-trap for unwanted intruders, and was forced to hand over the payment to Sir Samuel while suspended above the dragon pens.
In Snuff Vimes encounters him again, as a prime mover behind even greater crimes. Gravid Rust is capable of a certain greasy charm, and uses this to persuade people who should know better to join him in a series of escalating crimes and offences. Gravid was responsible for having goblins forcibly taken from Hangman's Hill, (which is a part of Vimes' estate), and shipped for use as slave labour on his tobacco plantation in Howondaland. He also used the same shipments of tobacco to smuggle a number of troll drugs into Ankh-Morpork along with the tobacco. These included the deadly Crystal Slam and others including Slab, Slice, Slap, and Slunkie.
Three years later Gravid attempted to move a second shipment of goblins to Howondaland, (having now worked many of the previously enslaved Goblins to death), also employing the ruthless killer Stratford to tidy away witnesses and lose ends. Here Gravid overreached himself, and by a lucky accident of chance Samuel Vimes was on holiday in the region to uncover this crime.
While most of Gravid's lackeys and smugglers were caught, and the corrupt magistrates removed, Vetinari was forced to point out to the enraged Vimes that slavery of goblins was not then a crime (goblins having being classed as vermin) and Gravid could not be brought to trial for his actions. While Diamond King of Trolls was demanding that Gravid be sent to him for trial (and near certain execution) for his role in the manufacture and shipping/smuggling of lethal troll drugs, the Patrician knew it would not be politically feasible to hand over the son of a prominent aristocrat for trolls to kill, regardless of his crimes. Instead Vetinari told Vimes that he would have to be content with Gravid being disinherited by his father, losing his position and title (having dragged the Rust Family name into public disgrace), and exiled to Fourecks for the foreseeable future. While this would be seen as a terrible fate for most aristocrats, Vimes saw it as 'a nice holiday in the sun' for someone who had trafficked in misery and suffering for profit.
In the end, they say all sins are forgiven. But perhaps not all...later, discussing the Rusts, Drumknott mentions that the Morporkian Embassy in XXXX employs a clerk named Arachne with a particular interest in venomous spiders. Vetinari must act in the dark and outside of the Law without sanction, a luxury he envies Vimes, to ensure that there is Justice for the goblins, compelled by having heard Tears of the Mushroom's harp composition, "Twilight Serenade".
The term 'Gravid' in Roundworld refers to when animals (fish or reptiles) carry their eggs inside of them. On the Discworld, the term also applies to swamp dragons, so probably has a different meaning. Then again, dragons are more or less reptiles and we don´t know that they don´t carry their eggs inside themselves.