From AMS Glossary
"The mean pressure distribution (at sea level) for a time interval during which the positions of the stationary (steering) cyclones and anticyclones and the steering within a special circulation region remain essentially unchanged."
A Grosswetterlage is thus a certain large-scale circulation pattern that is mostly synonymous with a particular long wave regime.
Baur, F. 1951. Compendium of Meteorology. p. 825.