I think everyone that thinks that gpit is "a fantastic spectator experience" just replays saber07 capping c with less than a second left on the clock and uses it as justification for the existence of a map.
I've never found gpit to be fun to play outside of crushing a team while defending and at the same point I'm sure it's the worst experience for the other team to have to sit through 12 minutes of getting fucking crushed into spawn. The reason it's tense and perhaps "enjoyable" for some people to watch is that the map is designed for 12v12 gameplay and when you limit it to 6v6 you basically michael bay every facet of the game because suddenly there is no forgiveness for anything. It's why pick classes are so popular and suicides so normal on the map because your 10% chance of working play can either fail for 8 minutes or you get the medic and set a 3 minute time. On the other hand, defense is painfully boring because you're running an engineer/heavy/pyro (and let's face it no one has ever found those classes "exciting" to watch) and you sit around with your dick in your hand until it's time to pray for airshots on the 4 people bombing your medic. Is this more dynamic than regular 5cp push/build/push gameplay? Possibly, but it doesn't change the fact that the very reason that it is tense is because you've made small mistakes explode into round winning losses which allows overall better teams to get upset fairly easily. People here critique a game like CoD because the game design is such that the skill gap is compressed so bad players never feel like they're getting stomped on and yet gpit does the exact same thing by allowing single picks to end rounds.
As for it being "a favorite spectator map", I seriously doubt the viewership and growth thereof for TF2 hinges on gpit being in the map pool. I'd be fine with another KOTH or a/d or a gpit edited to be sequential like plat suggested. Every map has potential to be a "story of the season" because at some point they all have been (Fanom on old viaduct, point and click on badlands, EG on granary, eMg on coldfront, EG on coldfront, dynamic on freight, loaded on follower, etc.) The point is that gpit is a seriously flawed map that has been kept around for tradition and while the classes chosen can be dynamic, the overall strategy has become very same-y.