The tag ref:at:gkz should be used for the Austrian municipality identifier (https://en.wikipedia.org/wiki/Community_Identification_Number#Austria). The number you provided is probably the VAT identification number of the restaurant. (https://en.wikipedia.org/wiki/VAT_identification_number)
The municipal identifier should also only be used on a municipal and not on normal entities inside of the municipal.

I think you added the tag ref:at:gkz to the wrong element. I removed the tag from Gasthaus Kremstalblick.