The Stands could just be effecting the soul using special powers like
Osiris which it's special power is to collect the souls of those of who lose.
That is true. Let’s look at every Stand that affects the Soul, then.
Osiris and his Bro: After someone loses the gamble, they lose their Soul. A Stand isn’t even involved, so we never see what happens to it.
SCR: Swaps people’s bodies by swapping their Souls. Stands follow suit, and it can manipulate someone Stand against them with this ability. Therefore, Stands and Souls in this example are interchangeable.
Doppio and Divolo: Unique example, someone with 2 Souls. This person also has 2 Stands. Interesting. So here, the amount of Souls = the amount of Stands. Just a coincidence? I think not.
Black Sabbath: Pulls your Soul out of your Shadow to stab it with an Arrow. If you have a Stand, it instead pulls it out. So here, once again, Stands = Soul.
I can’t think of any more examples that are canon, but feel free to provide them for me.
So frisk can still move and he is just moving gravity in a certain direction besides down? It still doesn’t seem like a massive hinderance since Frisk can still push against it.
Still doesn’t seem like enough to prevent someone like VI from just, pulling up.
Anyone who’s fought Sans can tell you that the Gravity
massively restricts movement. This is also a max-level Frisk, who one-shots everyone they see and massively up scales from 9-A whereas VI is just 8-C. Not much difference, and still massively restricts Movement either way.