The P1 vs. the Nordschleife is worth 20G and is unlocked by finishing the P1 vs. the Nordschleife Rivals event having set a lap time of 7:15 or better.

Enter the Rivals menu and choose the "Spec Challenge" rivals set and then the "P1 vs. The Nordschleife" event. You must be driving the McLaren P1 and get a lap time of 7:15. You may use rewind as often as you would like without penalty. Run through it one time to see what your driving style ends up with for an initial time and see how much you need to trim to get under the requirement. This is primarily an exercise in nailing corners, so if you don't get at least a "Good Corner" rating, I would suggest rewinding.

The P1 needs to be tuned to a maximum rating of R900 to qualify for this event. While you don't have a lot of options without exceeding R900, I would suggest weight reduction, a roll cage, and wider tires front and back as the best choices.