Based on the uniforms pictured and the machine gun, there are only two possibilities, Belgian or French. By 1917 (the year is written at the top of the first photo), the Belgian army had shifted to a khaki uniform cut in the style of the French uniform (though officer uniforms resembled that of British officers). The second photo contains the most clues as to the army of origin. The soldiers pictures including your Great Grandfather show them wearing Adrian helmets, the standard issue helmet for both the French and Belgian armies as of 1915. The same photo shows the soldiers posing with the Fusil mitrailleur modele 1915 CSRG or Chauchat, a notoriously poor quality light machine gun produced by the French and was supplied to the Belgians.

The second photo shows a hint of the right most soldier's boot and to me it doesn't look French to my eye. The French, post-1915, utilised puttees to protect the wearer's feet form water. These were strips of cloth that would be wrapped tightly from the ankle to just below the knee, sealing the top of the boot. The Belgians instead used leather gaiters that would be buckled around the ankle and lower leg to achieve a similar result. I think I can see a hint of leather gaiter in the group photo. That and the cut of the great coat strongly suggests Belgian to me but I cannot be 100% certain.