Grid had too much production and needed somewhere to put it, so they paid the GP to put the energy into GP's car.

On an electric grid, Supply and Demand must meet. When demand is low, you either reduce supply or increase demand and sometimes it's easier to increase demand.

Also, if you're supplying electricity, you may have reason to supply it even when the wholesale cost is negative. If you have production incentives not included in the wholesale price, you can be profitable at negative wholesale prices. (Things like green incentives, or base load incentives, or long term supply price guarantees)

If significantly reducing your output takes time or causes operational difficulties, it may be sensible to deliver at negative prices as you taper off. If you have a fuel shipment inbound and nowhere to store it, the negative wholesale price may be less expensive than cost to deal with the storage logistics.