GSP does not in any way, shape or form, indicate a players skill level. In fact, the higher GSP ranges are infested with some of the most lackluster, bad habit having, no combo or punish game having, walmart wifi having, bad strategy having, no parry tech having players who would get 3 stocked 3-0 in bracket offline. I've seen players with a flat 4 million GSP absolutely SHRED people with near 4.7 mil GSP.
The issue I have the most with GSP is, the players tend to NEVER rematch people who beat them, and only rematch people who lose to them, which of course, exaggerates their GSP by astonishing amounts. Most players have at least a spare 1 million GSP floating around off of that factor alone. The other issue is, rulesets. It does NOT matter what your rule set preference is, you WILL end up on RNG stages with smashballs and items, playing against the most laggy DDD on wifi possible. Being limited to only FD and BF is also a huge issue, as some characters don't want to go to one or the other, especially in specific matchups. That's the other thing, random matchups. You just might end up having to fight your worst matchup 10 times in a row because bad netcoding said so. Another issue, no mid-set character switches allowed during rematches, which is a HUGE flaw for the competitive elite smash scene.
To be blunt, 99% of players with high GSP have a fake GSP, built off the backs of casuals they rematch 19 games in a row, while avoiding rematches with the players who bodied them because they aren't carried by walmart wifi with heavies or counter spam.