Apart from his own compositions and his teaching responsibilities, during the late 1920s Gál, together with Mandyczewski, worked on the Complete Edition of the works of Brahms. Gál was responsible for the first 10 volumes, and Mandyczewski for the remaining 16. The edition was first published by Breitkopf and Härtel in 1926-7. Here the close friendship between them stood him in good stead. Hanna Gál recalled that

"At the time when the two of them were preparing the Brahms Complete Edition, and Hans was constantly busy in the archives of the Gesellschaft für Musikfreunde, which were looked after by Mandyczewski, they met almost every day, and Hans, who otherwise hated swallowing the dust in archives, was so entranced by Mandyczewski's intelligence, his knowledge and his rich store of memories, that the collaboration nevertheless gave him great pleasure." [Private correspondence, 10.10.1989.]

Hanna also recalled that on one occasion at this time, during a rather uncomfortable holiday in Aspang, with "no gas, a difficult cooker and above all an unreliable nanny",

"Hans escaped from the domestic misery as often as possible and climbed up the hill to the Mandys at Mönichkirchen . . . The old people were ill and very unhappy. Hans's visits did them good. It was the time when Mandy and Hans were working on the Complete Brahms Edition. There was no lack of topics of conversation, and that was good and useful for all concerned. One year later Mandy was dead." [Private correspondence, October 1989.]

Further scholarly activity involved editing volumes for the Denkmäler der Tonkunst in Österreich. Gál's contributions were the volumes on the waltzes of Johann Strauss (both father and son), published in 1926 and 1928. He undertook many other arrangements and editions, among them approximately 100 scores in the Philharmonia series, newly-founded by Universal Edition. His Anleitung zum Partiturlesen (Directions for Score Reading), likewise written for the Philharmonia series, had been published in 1923.