The versatile teenager collected a weak defensive header on the edge of the area following a German corner and took one touch before she rifled a shot through a crowd after 66 minutes.
Martina Voss-Tecklenburg's side saw most of the ball in Rennes, and Carolin Simon hit the crossbar in the opening period with what appeared to be a wayward delivery from the right flank.
However, China threatened on the counter and wasted two huge chances of its own in the first half. Yang Li took too much time with the goal at her mercy after 14 minutes, allowing Sara Doorsoun to deny her an opener with some last-ditch defending. Yang was perhaps more unfortunate with her second opportunity when her effort from the edge of the area bounced off the upright.
Gwinn, who will play for Bayern Munich in the 2019-20 season, is the first teenager to score for her country at the Women's World Cup since Ariane Hingst in 1999. She tried to add to her tally with a drive from an acute angle with 11 minutes remaining in the match, but it was corralled by goalkeeper Peng Shimeng at the second attempt.
Germany saw out the 1-0 win over the Steel Roses with some determined defending near the end, and will next face Group B rival Spain in Valenciennes on Wednesday.