Gerhard VI was born around 1367, the son of Count Henry II from the Rendsburg line of the House of Schauenburg and Ingeborg of Mecklenburg.

On 15 September 1386 King Olav III of Denmark enfeoffed him with the Duchy of Schleswig, after his uncle Nicholas had resigned from that function.

In 1390 Gerhard and his brother and uncle inherited Holstein-Kiel, including the merged Plön, whose line had been extinct in 1350.

After their uncle Nicholas had died in 1397 the brothers divided their possessions, the elder keeping Schleswig and Holstein-Rendsburg, and Albert II receiving Holstein-Segeberg as secundogeniture.

In 1403 Gerhard regained Segeberg by way of reversion upon Albert's death in action against Ditmarsh, thus ruling almost the whole of Holstein except of Holstein-Pinneberg.