He aided Overbeck in carrying out the frescoes of the Evangelists and Apostles at Castel Gandolfo, and at a later date, when Overbeck's strength was no longer equal to the task, Seitz, with the aid of his son, Ludwig Seitz, completed Overbeck's frescoes in the cathedral at Diakovar by filling the gaps with compositions of his own.
The name means of Gara / Gorja (modern village of Gorjani near Đakovo in Croatia), their estate.