In 1905, she sang at the Cologne Opera as the Queen of Sheba in La reine de Saba by Gounod, and as Isolde in Wagner's masterpiece.
For many years a picture of her performance was thought to show the play's author, Wilde in women's drag.
In 1905, she sang the lead role in Isidore de Lara's opera Messaline in Cologne, which was a great success for her.
Guszalewicz gave concerts at the opera in Frankfurt am Main (1907), at the Stadttheater of Bremen (1908) and at the National Theatre Munich (1910 as Salome).
Her repertoire included the roles of Leonore in Fidelio, Adriano in Rienzi, Venus in Tannhäuser, the title role of Ingwelde in Max von Schillings' opera, Elektra (opera) in the opera of Richard Strauss, Berthe in Le prophète by Meyerbeer, Santuzza in Cavalleria Rusticana, the title role in Mascagni's Amica, and Maria in A basso porto by Niccola Spinelli.