Yeah but they will be in big trouble in the race tho. It's all well and good dropping the drag by flattening the wings. But that means no downforce in the corners. We've seen over 1 lap the laptime is faster, so it's a good way to go right? So why haven't all the teams done this?

Because dropping downforce means the car will be sliding in all the corners. Not only is that harder for the driver to get that laptime every lap (they are F1 drivers, they should be able to do it), but it eats up the tyres. Sliding means overheating and gaining of the surface. In order to save their tyres they are going to have to slow down a lot in the corners, which means those straight gains go away very quickly. And they could easily require an extra pit stop compared to others.

For example somebody like Piastri who is compromised by the penalty going extremely long and stopping one time less might have a better race time and thus position in the end, even though all of his laps are slower than Gasley.

That said, it's still an amazing result by Gasley. Having the wings slimmed down is one thing, actually getting the lap time out of it is another.