He was a private investigator in new-York who moved to Paris in the 1920's. He tried to be an artist until 1925 when he was hired by Sophia Blake to investigate her brothers and her sisters death. During his investigation he encountered with the item called "Head of Baphomet" ( an item that is believed to give immortality to the one that owns it) .
In 1929 he moved to Prague and was hired by the prostitutes to investigate several murders.During his investigation it's revealed that the killer (AKA Mr. X)(Mark Ackerman (later revealed)) is actually an artist who manages to slip out by fleeing to America. Meanwhile Gustav's girlfriend Ida was brutally murdered causing Gus (along with Milena And Vladina) to flee to New-York where he starts a new life marrying Milena and having a child. The Last known info from him is that he stole some documents from the FBI in 1932.
Still Life 2
Gustav doesn't actually appear in this game but he's mentioned several times by his granddaughter Victoria McPherson