Land at Munich-Airport in the town of Erding, which is approx. 125 km (78 miles) from Garmisch-Partenkirchen. Either you can pick up your rental car here or you can take an MVV city train to Munich's main station, Hbf München, or to the smaller station, München-Pasing (transfer distances are generally shorter here). From there you can take a train to Garmisch-Partenkirchen (hourly intervals) and then continue on to Grainau by train, bus or taxi.
Or you can fly to the Innsbruck-Airport in Austria instead, which is approx. 65 km (41 miles) from Garmisch-Partenkirchen. Here you can pick up your rental car or continue by train to Garmisch-Partenkirchen and then by taxi, train or bus to Grainau.