Just stick to " Guten Tag" and you´ll be fine everywhere in Germany !
(even in Bavaria - where Gruss Gott basically is the "norm")

In the Black Forest region Gruss Gott is common in the southern part but Guten Tag is considered as equal everywhere!

So, it´s always correct to say Guten Tag !
You may not be able to recognize which phrase is the common one in the town you´re in (unless you get into a shop and the staff greets you with their local phrase)
Even for me as a native German "Guten Tag" is the norm !!!