Continuity mistake: Just after Norman has been made to shoot the German, he lies on the floor with his face buried in his arms. Camera cuts and his position has changed and you can now see his face. (00:39:20)
Continuity mistake: When Norm takes the picture off the post in the tank near the start, the way he holds it changes between shots from the extreme corner to further down. (00:15:40)
Answer: Gordo was making a point to Norman that the crew of the "Fury" had seen some horrible things during the Battle of the Falaise Pocket. After wiping out an entire German army there, they were tasked with putting wounded horses out of their misery. The point was that Norman was not there, and did not experience what they had, so Norman could not judge the tank crew's actions.
Scott215
Gordo brought up the horse thing because him and the other (original) crew members were not happy with Don and Norman enjoying a nice, quiet meal without them. He even say's "You weren't there" meaning Don sharing this with Norman when he hadn't yet been through much war time like all of them had, together. They also say "We weren't invited", "Why weren't we invited." The horse story was to get at Don for not inviting them and sharing it only with Norman.