The 19-year-old fired in from the edge of the box in the 66th minute to break stiff Chinese resistance in the Group B encounter.
China had almost stunned the favourites against the run of play in the first half when Yang Li was sent through after a German error but her shot was cleared when she looked certain to score.
Carolin Simon hit the woodwork with a cross minutes later but Germany's early dominance faded as the Chinese gained in confidence.
China earned another golden chance from a quick break, hitting the post in the 44th minute before Germany keeper Almuth Schult came to the rescue.
Gwinn then found just enough space in a crowded Chinese defence to fire home a cleanly-struck shot past keeper Peng Shimeng to settle the game.
Germany are now unbeaten in their last 13 matches, the longest run of any team taking part in the tournament in France. They next take on Spain on Wednesday, while China play South Africa on Thursday.
(Reporting by Karolos Grohmann; editing by Tony Lawrence)
