Neal O'Connor's AAOIGIWWI-Vol I Bavaria has this to say about it, pg 115 paragraph 2:
"Originally, all Gold Bravery Medals were made of the genuine material. As the war progressed, Germany suffered from a shortage of precious metals. By the fall of 1916, the Reichsbank was no longer able to supply the makers of orders insignia with sufficient gold for their needs. As a consequence, in an Imperial Decree on November 16, 1916, Kaiser Wilhelm II authorized the manufacture of orders and decorations that heretofore had been made in real gold to be made in silver-gilt. In the Hessenthal and Schreiber total of 1,198 Gold Bravery Medals, the breakdown they give is 745 in the real material and 453 in silver-gilt.
The first recipient of a Gold Bravery Medal in World War I was Obermatrosen Bruno Geist. He received it on November 6, 1914 as a crew member on the U-9....//...awards of the medals continued to be made retroactively after the Armistice. The last were approved in March 1920."