From Vienna via Semmering-pass to the Mürztal and via Niederalpl to Gußwerk, then proceed towards the village Wildalpen. 4km before Wildalpen there is a big valley (Brunntal) on the left side (there is a lake in summer (Brunnsee), but it's covered with snow in winter). There is a tiny road on the left side. Turn into this road and follow it (approx 500m) to a traffic sign, which doesn't allow you to proceed. There is a parking lot for approximately 10 cars.
The downhill follows mostly the ascent. The only difference is, when a 30° steep downhill is not satisfying for you, then you may ski down from Griesstein more or less directly into the upper end of the gorge. This way you may have for approximately 350 feet height difference a steepness of up to 40°.
Normal ski mountaineering equipment. A GPS and/or compass and a good map!
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