Gueller's specialisations encompass the Classical, Romantic and late Romantic repertoire, as well as large-scale works orchestrated for smaller orchestras. He is also highly regarded as an interpreter of contemporary work, having introduced many pieces to audiences. In Adelaide, Barcelona, Cape Town and Halifax in particular, he has also introduced the works of many local composers, with festivals of music by Canadian composers Christos Hatzis and Malcolm Forsyth. He is involved in the commissioning of works by Canadian composers, most recently a piece for accordion by Brian Current, premiered by Joseph Petric, and Hurricane Juan Concerto for piano, composed and premiered by Peter Allen, with an orchestral work and one for tabla in the pipeline. He also conducts large choral concerts, from Rossini's Stabat Mater to Messiah and Beethoven's Symphony no. 9, as well as concerts of opera arias. He is an acknowledged interpreter of Mahler and Bruckner. Gueller conducts lighter music as well as a multimedia presentation of Holst's The Planets. His interpretation of Mendelssohn's A Midsummer Night's Dream with choir, soloists and narrator was critically and popularly received, as are his unusual “Musically Speaking” concerts in which he combines the music of, for instance, Tchaikovsky with readings from his letters, and concerts that include the composer's sketches for Beethoven's Symphony no. 5 before playing the final version.

He has received critical acclaim for his concert version of the Strauss opera Elektra with Cape Town Opera and the Cape Town Philharmonic Orchestra, The Nutcracker and Romeo and Juliet ballets with Symphony Nova Scotia in Halifax, and has premiered a new ballet, Anastasia.