After he graduated from Westminster in 1736 there is no record of Gage's activities until he joined the British Army, eventually receiving a commission as ensign. His early duties consisted of recruiting in Yorkshire. In January 1741 he purchased a lieutenant's commission in the 1st Northampton Regiment, where he stayed until May 1742, when he transferred to Battereau's Regiment with the rank of captain-lieutenant. Gage received promotion to captain in 1743, and saw action in the War of the Austrian Succession with British forces in Flanders, where he served as aide-de-camp to the Earl of Albemarle in the Battle of Fontenoy. He saw further service in the Second Jacobite Uprising, which culminated in the 1746 Battle of Culloden. From 1747 to 1748, Gage saw action under Albemarle in the Low Countries. In 1748 he purchased a major's commission and transferred to the 55th Foot Regiment (which was later renumbered to the 44th). The regiment was stationed in Ireland from 1748 to 1755; Gage was promoted to lieutenant colonel in March 1751.