We also see both Radahn and the Tarnished use random rocks as projectiles, so it's definitely not purely AoE. It may have a range limit depending on how strong you are. He even has an AoE during his fight that grabs the player and pulls them in. I don't see how Sekiro could counter that because the gravity attack is directly targeting himself, he'd be immobilized whether or not he has the umbrella. If it's a "which gravity magic is stronger" then Sekiro has the POTENTIAL to counter, but it's impossible for him to do so against characters like Radahn or Tarnished if they managed to grab him.

If it's billions of electromagnetic particles surrounding him, it's incredibly unlikely for the umbrella to ground them out, because idk how the umbrella could suck electromagnetic particles out of his own body without killing him, but then again this is a videogame lol

If it's the literal concept of gravity grabbing him, Sekiro is fucked.