The striker was called up on Friday ahead of the game at the Ernst-Wagener Stadium in Steinburg. Gustav started the match which saw few real chances.
The best opportunity of the first half belonged to the Danes; Albert Grönbaek's header bringing a good save from goalkeeper Luca Unbehaun. Lazar Samardzic had the ball in the net for the hosts with a quarter of an hour to play but it was ruled out for offside.
The cap was Gustav's 38th at youth level for his nation, during which time he has scored nine goals.
The international action tonight sees Bryan Mbeumo and France's Under-21s travel to Azerbaijan in UEFA European Championship Qualifying, kick-off 6pm.