You should greet people with Grüß Gott in Austria and Bavaria
The widespread German greeting "Guten Tag" is considered prim and distant in Southern Germany and Austria and it might lead to misunderstandings. People use "Grüß Gott" instead, which literaly means "May God greet you". Also, instead of the informal greeting "Hallo", most of Southern German speakers, use the word "Servus".