I've taken many screen shots, all online advice from Z3c users say that screen shots are saved in the gallery, but there is no "Gallery" app. Only Google photos, and it doesn't show up there?
Open Google Photos. Press on the 3 bars to the top left corner. Navigate down to Device folders. There you will see Camera, Screenshots and any other applications that receive photos, for example, WhatsApp, Plume, Facebook etc.