Basically, GPS satellites need ridiculous precision in their clocks to make navigation work. They all have atomic reference clocks to accomplish this.
Since GPS satellites are already broadcasting a ridiculously precise signal to the entire planet for free, it makes a lot of sense to use that signal when you need very precise time.
Though in cases like this where critical infrastructure is involved, I'm confused why they don't have an adequate clock source locally. You don't usually rely totally on GPS, you use it as a reference to check your local clock against.
More to the point, all GPS satellites broadcast the time - the receiver takes the multiple (different) times from multiple satellites and the satellite's known location to determine the receiver's location.
It’s related to power generation. Finding line faults, keeping equipment at 60hz or 50hz whatever they use there.
They need precision location information for multiple power plants. They can do without it but you end up with dirty power which most equipment doesn’t like very much and some will shut off and not work at all or break.
You can make a reference oscillator almost as precise as an atomic clock with a temperature compensated crystal oscillator. They cost tens of dollars.
We've also been synchronizing grid generation since long before spaceflight. It seems that the core problems here have been solved a long time ago.
Then again, I don't know much about the grid at this scale. I very much doubt that a few PPM phase difference would actually matter, but I don't really know.
Either way, it still seems insane to rely on GPS for your sole source of truth, even with its remarkable reliability.
Since GPS satellites are already broadcasting a ridiculously precise signal to the entire planet for free, it makes a lot of sense to use that signal when you need very precise time.
Though in cases like this where critical infrastructure is involved, I'm confused why they don't have an adequate clock source locally. You don't usually rely totally on GPS, you use it as a reference to check your local clock against.