Gassner was born on 12 April 1876 in Triesenberg as the son of Landtag member by the same name and his mother Maria Anna Bühler as one of two children.

[2] As a member of the Landtag, Gassner wrote contracts, wills and petitions to the government for many people and provided information in legal matters regarding Triesenberg.

[3][4] From 1922 to 1927 he was president and from 1927 to 1930 Vice Chairman of the board of directors at the Lawena power plant in Triesenberg, of which he had advocated for its building.

[2] This resulted in the 1930 Liechtenstein by-election and granting the Progressive Citizens' Party an absolute majority in the Landtag.

[1] Gassner married Katharina Beck (7 January 1878 – 9 August 1923) on 5 February 1900 and they had six children together.