Gil's doing it for Bang's own good… not to mention everyone else in Europa.
The inoculation draught is a potion which in theory renders the imbiber immune to possession by slaver wasps. It is created by Tarvek, who details its method of creation in a set of encrypted notes. (When exactly he found time to do all of this is not made clear.) During the Siege of Mechanicsburg, while he and Tarvek are on Castle Wulfenbach, Gil purloins the notebook and ejects Tarvek, Othar Tryggvassen (Gentleman Adventurer) and Captain Vole off the airship, sending them back to the city below via his experimental airship. With the "help" of Bang, he brews up ✣ a batch of the stuff in his secret lab and tests the result ✣ on her. Ingredients for the formulation include silver of vixonite, distillation of Monahan's venefirous mus elixir, and calixia oil. (Also mentioned but not as part of the formula: Pellicax's twist and St. Michael's toes.) We get to see the initial impressive side-effects before an enraged glowing Bang knocks out Gil and the scene cuts away.
When Gil wakes up ✣ , he is being detained by his father, who declares that Gil has long been a revenant, and adds that Bang appears to have been "poisoned" by the formula. Subsequent event indicate that this is all a blatant lie on Klaus's part; at the very least, Bang recovers without any sign of long-term damage.
Following the timeskip and Gil becoming Baron, most, if not all, Wulfenbach minions and staff have been inoculated with the stuff, but especially foreign operatives like Woger and Susa. Susa had thought herself too small a target for the wasps to hit ✣ , but is glad she has been inoculated when she sees the mosquito-sized ones produced by the new portable Hive engines.
The Novels[]
The creation scene as portrayed in Agatha H. and the Siege of Mechanicsburg is basically the same as the comic, but goes into a bit more detail, ending with Bang trying to decide whether or not to force the remaining dose of the draught down an unconscious Gil's throat.The scene when he wakes up hints that she did indeed feed it to him.