Only for the very short term firming today, i.e. for frequency regulation. For days-weeks storage you need much lower cost/kWh of storage. There are contender technologies but nothing at scale deployed as far as I know (other than pumped hydro, which is geographically limited).
For days/weeks storage -- it's not a problem yet. We already have plenty of natural gas plants in the US. I'm perfectly happy, environmentally, if we have to run natural gas plants two weeks a year but it means that we get to 80 or 90% clean energy a decade or two faster than Plan B.
Natural gas isn't the only way to avoid plan B (assuming that refers to giving up on controlling global heating).
In CA, natural gas peakers emit 60M metric tons of CO2 per year (out of a statewide 420M metric tons), so a significant portion. Much of the non-peaker C02 is also natural gas based.
In addition, as more natural gas in baseload gets supplanted by renewable and storage and maybe small modular nuclear, the utilization of natural gas will drop, resulting in that generation becoming stranded assets (similar to what is happening to coal now).
This is also before accounting for all the other ways power could be stored batteries, demand shaping and storing syngas.
But what percentage are close enough to existing transmission? Perhaps some, but I suspect many suitable locations will be remote enough that new transmission needs to be built. It might make economic sense to do so in some circumstances, but not in others.
In contrast, grid scale chemical storage (flow batteries, metal-air, etc) could be easily placed at existing fossil fuel plants and use the existing grid interconnects.