He was able to finance a study trip to Paris by selling a portrait of King Friedrich Wilhelm III of Prussia to the royal family.

While there, he acquired more skill in the manipulation of light and atmosphere, and was inspired by the magnificent vistas of medieval buildings to devote himself almost entirely to painting vedute.

Over the next ten years, he devoted himself to documenting the Biedermeier style buildings of Berlin and, with royal customers in mind, produced a series of scenes depicting the castles in Bellevue, Charlottenburg and Glienicke.

His successor, Friedrich Wilhelm IV, preferred Italian-style paintings with Greek landscapes and bought very little from Gaertner who, without the income from his principal client, soon began to have financial difficulties.

As a result, Gaertner traveled to villages and towns throughout Prussia, making watercolor sketches, including scenic views meant to be sold on his return to Berlin.

He began to turn away from architecture, producing romantic scenes full of steep cliffs, Roma, ruins, and oak trees, but never restored that patronage.