I've been unable to find anything about this handsome Prince Gorchakov except the dates of his birth and death - 1875 and 1916 - and that in 1904 - the year this portrait was painted - he married Daria Mikhailovna Bibikova (1883-1962), a lady-in-waiting to Empress Alexandra Feodorovna. The year of his death and his relative youth - he would have been only forty-one - leads one to wonder if he may have been a casualty of World War I. (https://curiator.com/art/nikolay-bogdanov-belsky/prince-alexander-konstantinovich-gorchakov)