From 2004 to 2009, he continued his studies with Rudolf Piernay at the Hochschule für Musik und Darstellende Kunst Mannheim and at the Guildhall School of Music and Drama of London, where he graduated with distinction.
In the realm of music theatre, Gädker gave guest performances on the stages of Braunschweig, Leipzig, Frankfurt and Mannheim, where he was among others Count Almaviva (The Marriage of Figaro), Aeneas (Dido and Aeneas), Schneck (Der Vogelhändler), Notario (Gianni Schicchi) as well as in other roles in world premieres of the opera Wasser by Arnulf Herrmann and the chamber opera Neumond by Lucia Ronchetti.
In 2013 he appeared in the title role of Shostakovich's opera Das Märchen vom Popen und seinem Knecht Balda[5] in a production of the Konzerthaus Berlin and from 2014 he could be heard for two seasons in Shakespeare's The Tempest (direction Calixto Bieito, music by Henry Purcell) at the Mannheim National Theatre.
In the 2014/2015 season, Gädker performed in an opera pastichecio after Wilde's The Canterville Ghost with music by Rameau, Purcell and others at the Musiktheater im Revier (MiR) Gelsenkirchen.
In 2011 Gädker founded the international song recital series "klangwerk LIED" Freiburg together with the soprano Katharina Persicke[6] and the pianist Nicholas Rimmer.