Gustav Fiers is a senior executive at Oscorp Industries and a high-ranking council member for the terrorist organization HYDRA. For decades, the wealthy and misanthropic Fiers controlled a secret organisation formally allied to HYDRA with the sole purpose of eliminating key targets opposed to their plans during various key moments in the last 50 years, having been made aware in that time about HYDRA's true ancient origins involving the Inhumans. Fiers also had a lethal reputation in the criminal underworld, known for conducting elaborate plans to increase his own wealth in the process. Gustav eventually learned that Richard and Mary Parker, two mercenary agents under his employ, were actually S.H.I.E.L.D. agents sent by Peggy Carter to investigate the possibility of HYDRA's resurgence within the military organization. When the Parkers were hired by Albert Malik, Johann Schmidt's replacement for the title of the Red Skull, to begin work on the Sinister Six - Spider-Man initiative as a countermeasure against the Avengers Initiative, Fiers informed his associate of their disloyalty and of the possibility of them unraveling the HYDRA conspiracy years before it was time to do so. Malik thus hired Gustav Fiers' younger brother, the Finisher, to kill the Parkers. He succeeded in doing so but at the cost of his own life when the plan they were on crashed to the ground.
Upon hearing of both the deaths of the Parkers and his brothers, Fiers began creating an elaborate scheme to exact revenge by proxy on the Parkers' extended family for the two agents nearly ruining the grand vision that HYDRA had in store for the world, eventually targeting their son, Peter Parker, when he came of age and into the prominence of the superhero committee. As part of the revenge plot, Fiers (now calling himself "the Gentleman") began recruiting various Oscorp Industries members and miscellanious street thugs to join the Sinister Six, with the various members composed of Doctor Octopus, Electro, Mysterio, the Vulture, the Chameleon, the Rhino, Kraven the Hunter, and the Sandman, with any available member of the Osborn family leading the pack as squad-commander Green Goblin. Doctor Curt Connors was also intended to join the program, having been informed of the truth behind the Parkers' demise beforehand by Fiers and lured to his side as a secret consultant in creating the cure for the Globulin Green disease in exchange for being given back a replacement for his severed left arm, but his failure to jump-start events as the Lizard-Man led Fiers to simply fire Connors from his position, but not before receiving a reluctant assurance from Connors that the plan to build the Sinister Six was still a secret to Spider-Man.