[2] Born in the family of Prince Peter Ivanovich Gagarin and Anna Mikhailovna (1715–1782), daughter of Kiev Governor-General Mikhail Leontiev.

In 1771–1772, under the name of Penzin, with his relative Alexander Kurakin and Nikolai Sheremetev, he made a Grand Tour of Europe (Leiden, Antwerp, Brussels, Calais, London, Paris) to supplement his education.

In January 1773 he returned to Russia; half a year lived in St. Petersburg, since July 1773, as a volunteer, took part in the Russo-Turkish War.

According to Fyodor Rostopchin, "this prince Gagarin was a business man, but a depraved snare, entangled in debt and lost all reputation".

[4] He spent the last years of his life in his Bogoslovskoye village in Dmitrovsky Uyezd of Moscow Governorate, where he was buried in the local church.

In 1775, he entered into marriage with Praskovya Fedorovna Voeikova (October 25, 1757 – July 11, 1801), daughter of the Kiev Governor-General, General-in-chief Fedor Matveevich Voeikov and widow Anna Ivanovna Zherebtsova.

[3] Princess Praskovya Fedorovna died aged 44 in Serpukhov; She was buried under the Intercession Church in Novospassky Monastery, in Moscow.

Archbishop Theophylact (Lopatinsky) dedicated his essay to Gagarin, written back in 1787, "The Mirror of the Hottest Spirit to the Lord God".