Gundersdorf is a former municipality in the district of Deutschlandsberg, in the Austrian state of Styria. It lies in the western part of Styria, within the Deutschlandsberg district, an area of rolling hills and farmland in southern Austria.
Since the 2015 Styria municipal structural reform, Gundersdorf has been part of the municipality of Sankt Stefan ob Stainz. The reform merged a number of small municipalities across Styria into larger administrative units, and Gundersdorf ceased to exist as an independent municipality at that time.
Its administrative association with Sankt Stefan ob Stainz places it in the Stainz area of the district, named for the market town of Stainz nearby. As a former municipality, Gundersdorf is today a locality within the larger commune rather than a separate governing entity.