David, to be honest with you, I “doubt” that any of the soldiers in this photo were from Artillery units.
The reason why, Artillery units moved “with their Artillery”......meaning that when they moved, they travelled with their guns, equipment, ammo etc etc. They moved as a unit. In this photo, of course we can’t see any guns, or anything that would indicate that these are “gunners”.
So, unless your “positive” the photo is of you grandfather, I’m not sure if any of these men would be gunners.
And, finding someone that knows their relative is in this photo may be very very difficult.
But, maybe one of our other members can give an opinion or answer on this.
And of course, it’s my opinion and I could be wrong