Isachsen married Signe Amalie Eide (1876–1911) in 1903.
Signehamna harbour in Svalbard is named in her honour.
 They had three children (Fridtjov, Nils, and Gerd).
His second marriage, in 1916, was to Marie Sophie Louisa Steenstrup (1884–1958).
They had five children (Kjell, Inger, Karen, Odd, and Finn); Odd Isachsen is still living.
Through his second marriage, Gunnar Isachsen was a brother-in-law of Hjalmar Steenstrup.

In 1903, he was knighted 1st Class of the Order of St. Olav, and was commander of the Cross 2nd Class in 1931.
He held the King's Medal of Merit in gold (1912), as well as a number of foreign orders and medals.

From 1911, he lived on his farm Vardeborg, under the Vardåsen, the highest point in Asker, Norway.
Isachsen died of a heart attack in Asker in 1939.
Isachsen married Signe Amalie Eide (1876–1911) in 1903. Signehamna harbour in Svalbard is named in her honour.  They had three children (Fridtjov, Nils, and Gerd). His second marriage, in 1916, was to Marie Sophie Louisa Steenstrup (1884–1958). They had five children (Kjell, Inger, Karen, Odd, and Finn); Odd Isachsen is still living. Through his second marriage, Gunnar Isachsen was a brother-in-law of Hjalmar Steenstrup.
In 1903, he was knighted 1st Class of the Order of St. Olav, and was commander of the Cross 2nd Class in 1931. He held the King's Medal of Merit in gold (1912), as well as a number of foreign orders and medals.
From 1911, he lived on his farm Vardeborg, under the Vardåsen, the highest point in Asker, Norway. Isachsen died of a heart attack in Asker in 1939.
Gunnar Isachsen