After a busy 1978 season, Gross was traded by the Cubs on February 23, to the Philadelphia Phillies with Manny Trillo and Dave Rader for Jerry Martin, Barry Foote, Ted Sizemore, Derek Botelho and Henry Mack. Gross established himself as a key platoon outfielder and first baseman for the Phillies, getting to play in the 1980 National League Championship Series (he went 3-for-4—all pinch hits—with one RBI in four games), the 1980 World Series (0-for-2 in four games), the 1983 NL Championship Series (0-for-5 in four games with a run scored), and in the 1983 World Series (0-for-6 in 2 games). Gross was an invaluable pinch hitter. In 1982, he led the league in pinch hits with 19.