> What's a "GW of battery storage capacity"?

This is the maximum amount of power it can dispatch. I think they use it intentionally so they can compare it to other generation sources.

For example this spreadsheet [1], with data for 2023, contains figures for "power capacity megawatts" and "energy capacity megawatthours".

1. https://www.eia.gov/analysis/studies/electricity/batterystor...