I did the same calculation. Gravity energy storage is a joke. Came to similar conclusions when running numbers on hydrogen-powered vehicles.

Pumped hydro storage and flywheels are cool but ultimately battery storage, distributed everywhere, will win.

Gravity storage is cool when nature has already made most of the work, I/E pumped hydro where nature has already built this huge canyon with a river in the middle just waiting for someone to put a dam at the end.