In 1619 Gage enlisted as a gentleman pikeman in the Army of Flanders and initially served in the garrison of Antwerp. In 1622 he obtained a commission as captain of a company in the regiment of the Earl of Argyll. He distinguished himself during the Siege of Bergen-op-Zoom (1622) and the Siege of Breda (1624). In 1625 he was serving as captain in Sir Edward Parham's regiment, but during the Anglo-Spanish War (1625–1630) he returned to England rather than serve against his native country. During these years his translation of Herman Hugo's "Obsidio Bredana", a Latin account of the Siege of Breda, was published in Ghent by Judocus Dooms under the title "The Siege of Breda".