Carl Johan Louis De Geer af Finspång (born July 13, 1938 in Montreal, Canada) is a Swedish artist, writer, musician and friherre (baron) of the De Geer noble family.
Gustav's namesake great-great-grandson, barrister Gustav Hamilton, then of Dublin, laid in mid-19th century claim to the baronial title and seat of Deserf in peerage of Sweden, as he was de jure 9th friherre of Deserf.
On 22 November 1747 he married, in Saint Petersburg, Maria Aurora von Mengden (born 1720), daughter of friherre Magnus Gustav von Mengden (1663–1726), former Lord Marshal of Swedish Livonia.