yea, my first comment is that generally none of those fish can live harmoniously with goldfish for a number of reasons.
However, from the pics i would guess it is the gar. Both oscars could probably swallow the golds from the look of the pics, though i could be wrong. A 20 inch clown knife could swallow both of those. the pictus would harrass and fray the fins and try to chomp on the golds if possible.
I would say the first pic is consistent with a gar tearing away at the gills and perhaps the tail, or maybe the pictus joined in on the fun. Or maybe the oscar grabbed the tail and the goldfish escaped. I have seen similar injuries infliced by oscars on larger fish like aro's.
the second is a tough call, especially considering that many fish will take shots at a dying fish. It might be consistent with the gar clamping down on the fish sideways, and the goldfish initially escaping.
my gar used to chomp on goldfish intially, and scales would be immediately displaced. Blood would generally appear, but the gar would hold them and they would sometimes escape prematurely, scaling a large portion of the rest of them.
Just my humble opinion. I hope you find your culprit, but I would not be suprised to see more goldfish fatalities in that setup.
