From Salzburg approx. 1 hour:
South on the Tauern Autobahn (motorway) to Bischofshofen, Gasteinertal exit, then highway B 167 to the Gastein Valley.

From Innsbruck approx. 2 hours:
Autobahn via Wörgl, highway B 312 via St.Johann/Tirol to Lofer, B 311 via Zell am See to Lend, B 167 to Gastein or on the autobahn via Kufstein - Salzburg - Villach, Gasteinertal exit, B 167 to the Gastein Valley.

Coming from the south, drive via Villach, Möllbrücke and Obervellach to Mallnitz, at which point you will be able to enjoy the convenience of motorail service to Gastein.

... and with the route planner it's easy:

Simply enter your hometown and start the route
planner. You will receive detailed directions for
Bad Hofgastein.

Once you have reached Bad Hofgastein, take the "West" approach road (at the Schlossalmbahn),
follow the road keeping left, cross the bridge
(after approx. 300 m) and then take the first left -
into Dr. Zimmermannstrasse. Our hotel is on the
left (after approx. 150 m).