OK yeah, it doesn't work on 6.1 (my bad, you can't update to 6.2 with a cart, only 6.1)
I did a Gateway downgrade to 4.2, then used my retail copy of Pokémon Y to upgrade it back to 6.1.
Tried both the 4.x and 9.x MSET exploits and neither worked.
So if I have to downgrade the system settings app *anyway*, I mayaswell just keep my sysNAND on what it had from the factory (7.2), since the Gateway downgrade process is rather sketchy.
So... the million-dollar question: how do I go about downgrading the system settings app? I tried the "Install NVRAM" option from Gateway's launcher but that didn't do anything.