He worked in various legal roles, including contributing to legislative matters at the Ministry of Justice and helping draft a Swedish-Norwegian water rights convention.

He deepened his familiarity with the French language and culture through studies at the University of Grenoble, and he maintained a lifelong interest in the humanities in general.

Gyllenswärd was particularly captivated by historical works, especially those of a biographical nature, and his knowledge of past eras, both their history and their stories, was extensive.

His active service in the Court of Appeal was interrupted as early as 1925 when he became an acting judge referee and later became attached to the Ministry of Justice's legislative department, where he served as an extraordinary member from 1927 to 1929.

Gyllenswärd served as secretary to the experts investigating the issue of amending the 1905 convention with Norway regarding shared lakes and waterways from 1925 to 1928.

[4] On 3 July 1919 in Bergunda Church [sv][6] in Bergunda [sv], Växjö Municipality, Kronoberg County, Gyllenswärd married Countess Anna Aurora Maria Posse (30 June 1894 – 18 March 1967), the daughter of Count Knut Arvid Posse and Baroness Sigrid Elisabeth Carolina Gustafsdotter Leijonhufvud.