Well I don't have video examples, but a horse can be fast but not have a great gallop.
To me, a good/great gallop is naturally uphill and should feel and look effortless. You barely hear the horse as they float past you and the rider can rebalance by sitting up a stride out from the fence. The stride covers a lot of ground so you need fewer strides to get from A to B - small horses can have big strides/big horses can have small strides.
The not so good gallops are strung out, on the forehand and heavy - they are fast b/c the legs move fast, rather than cover a lot of ground. You just don't get the sense that the horse is ready to jump something without a lot of re-balancing. (rider sits down 6 strides out and the half-halts remove molars). And you feel them coming because the ground shakes.
Not the best video because Kate never really let him open up at this event but this horse got a perfect score for his gallop when he competed in the YEH championships.
He was just coming back from an injury in this event so still wasn't super fit yet....but he does have a very good gallop--you can catch a quick sense of it in the video in spots where she lets him go on (toward the end). It is really fun to sit on...but he can get really strong.
In my experience competing in the YEH classes...they want a free big stride. You really need to open them up and let them fly to get the good score. They are not judging the adjustability of their gallop yet. So it is more the quality of the gait than really whether the horse will be fast on course (that is more adjustability). A horse with a big gallop like Muggle is what they were scoring well. He's he type of horse you want on a big open course....he would have loved phase B of the old long format....