In Smash Ultimate, my highest GSP is 2,602,353 (not very impressive, I know). Characters I haven’t played as have a GSP of 2,499,349, which is quite close but still about 100,000 lower. How is this number determined?
From what I understand, GSP is started at the mid-point. Or somewhere close to there, plus or minus some hidden variables that might be factored in. According to this site the estimated max GSP is around 5.25 million, meaning the dead center would be around 2.625 million.
All unplayed characters will float around the midpoint for GSP because rather than starting players from the bottom, they prefer that players either fall/climb from the middle. If you haven't touched them, then they stay in that same neutral state. I bought the game on launch, and still haven't played a handful of characters online. Their GSP has risen from below 1 million to roughly 2.5 million like yours currently are without ever being touched. How is that possible?
Well the way GSP works is more like a leaderboard number rather than a ranking system. The person with a GSP of 1 is the absolute worst player in the world, and a player with a GSP of 5.25 million whatever is most likely the best player in the world. If your character stays untouched, they float or rise depending on how many players there are. If there's 2 million players, then 1 million will be the dead center. If there's 5 million, then 2.5 million will be the dead center, and so on. Similarly, if your GSP is 100,000 with Captain Falcon - if someone else's GSP drops below 100k, you will rise to 100,001 despite not actually playing a single game to earn a higher spot.
Which theoretically means that if somehow every single person who played, say Duck Hunt, lost every single game they played, your Duck Hunt with one win would be the #1 in the world.