GDP is equal to the Wages + Profits at every stage of production.
Stage 1: Unnamed company sells 1M as profit in car parts to Car Motors (If the goods are imported then that 1M does not factor in, also assuming it was pure profit)
Stage 2: Car Motors Turns the car parts into cars
500K is given out as wages to workers
1M is made in profit for the company.
Total GDP gain: 2.5 M.
If a tsunami destroys all the cars it means that the cars cannot be sold and the profit for the company is $0.
Thus GDP gain is 500K in wages paid out to workers.
The reason you are getting conflicting answers is due to a lack of information.
GDP = Investment (I) + Government Spending (G) + Consumption (C) + Exports (EX) - Imports (IM)
It is always assumed that Investment = Savings (Look up the IS relation). So if the workers were savings all of their earnings I = 500K and if they spent it all in the economy C = 500K. Either way you have 500K in GDP gain from wages. Since the cars are being exported EX = 2,000,000 for a total of 2.5M assuming the export occurs. If it doesn't then only the wages are counted.