Alpgau (also Albgau) was a territory of Alemannia and the Duchy of Swabia during the 8th to 12th centuries. It comprised St. Blaise in what is now the Waldshut district of Baden-Württemberg.
It is not to be confused with Alpengau, now known as Allgäu.
References
Alpgau Wikipedia(Text) CC BY-SA