Reith is a common town name in Austria and I think Philip has the wrong one. Your reference to Schloss Matzen leads me to believe it is Reith-im-Alpbachtal. Still 7 changes from Gimmelwald but only 7 hours 40 minutes. You can look at the schedule at www.bahn.com. Use Brixlegg as your end point station.
If you are staying at the Schloss Matzen, a complimentary local transport card is included. There is a local bus from Brixlegg station (or Jenbach station) to Schloss Matzen.
Moving on, you can take a local S-bahn train to Kufstein, where using the Bayern Ticket, you can get to Scliersee in under 2 hours. The train changes in Munich. Or you can rent a car once you get to Munich.