This is correct for early war field gunners. As far as I am aware, only Alpine gunners received back packs as the field gun crew sat on the gun/limber.
Edit. see, for example, the photo of Royal Bavarian 10th Field Artillery Regiment (August 1914) in Kraus, J., The German army in the First World War: Uniforms and Equipment, 1914-1918 (Vienna, 2004), p. 428.
I suspect they may have slipped them off for prolonged fire missions.