On the 31st of October 1797 Gustavus married Frederica Dorothea, daughter of Charles Frederick, grand-duke of Baden, a marriage which might have led to a war with Russia but for the fanatical hatred of the French republic shared by the emperor Paul and Gustavus IV., which served as a bond of union between them.
On the 21st of August 1745, by the command of his aunt, he married the princess Sophia Augusta Frederica of AnhaltZerbst, who exchanged her name for that of Catherine Aleksyeevna.
His unfortunate first marriage with his cousin Charlotte Frederica of MecklenburgSchwerin was dissolved in 1810.
In 1791 he married Frederica Wilhelmina, daughter of Frederick William II., king of Prussia, thus cementing very closely the relations between the houses of Orange-Nassau and Hohenzollern.
His father was Christian Augustus (1673-1726), duke of Schleswig-HolsteinGottorp, bishop of Lubeck, and administrator, during the war of 1700-1721, of the duchies of Holstein-Gottorp for his nephew Charles Frederick; his mother was Albertina Frederica of BadenDurlach.
Man, without energy or resolution, and he allowed W himself to be entirely led by his old guardian the duke of Brunswick, and by his wife Frederica Wilhelmina of Prussia, a woman of marked ability, to whom he entirely deferred.
Her only daughter Frederica had contracted in 1748 an unhappy marriage with Charles Eugene, duke of Wurttemberg.