Fixing panel gaps - GIF

[Saltfever]

I wonder if that's the way the guys at Koenigsegg do it too?

Yes, but they use a precision calibrated gauge made from zerodur by Heidenhain and certified with a CMM in an environmentally controlled chamber. The gauge transmits data via bluetooth to a SPC (statistical Process Control) system that Koenigsegg uses in presentations at marketing meetings. I just use a paint stirring stick.