Born in Stirling, he was the son of James Christie (1695-1745), a wealthy merchant and manufacturer with branches in Glasgow, London and Baltimore. Gabriel's mother, Catherine Napier, was the daughter of Francis Napier (d.1713), of Craigannet; Provost of Stirling and a grandson of the distinguished John Napier. Christie was educated in Stirling, Glasgow and England. Gabriel Christie entered the army as an ensign in 1742 under the patronage of his uncle, Lt.-General Robert Napier (1708-1766), Chief Staff Officer to the Duke of Cumberland at the Battle of Culloden.