Gethin made his debut in F1 for McLaren at the 1970 Dutch Grand Prix, replacing the team's founder, Bruce McLaren, who had been killed at the Goodwood Circuit earlier in the month. He joined a three car team which also included Andrea de Adamich and Dan Gurney. He qualified strongly at Zandvoort, taking 11th, 2.110s behind Jochen Rindt's pole time for Lotus. He was nine-tenths of a second ahead of his more illustrious but ageing teammate Gurney. He engaged in a good fight early in the race with Henri Pescarolo's Matra, but soon got the better of the Frenchman and was still running in 10th when he had an accident on lap 19.