1. Of an opponent or enemy, to advance toward someone or something's position. The enemy forces are gaining ground on us. We need to move out! With the finish line only meters away, the runner turned back to see his rival gaining ground on him.

2. To encroach on the success or popularity of someone or something; to advance or prosper at the expense of someone or something else. The fledgling social media website has been gaining ground on the leading platforms in recent months. The underdog candidate's grassroots campaign helped her gain ground on her opponent, making this election much closer than anyone would have anticipated.