I found a very nice background explanation in The Presbyterian Hymnal Companion (1993), by LindaJo H. McKim

559 We Gather Together
Tune: KREMSER
This folk hymn dating from the seventeenth century was first written during Holland's struggles for independence from Spain. The political environment played a role in the writing of the text. Frederick Henry, prince of Orange, had just assumed leadership of the Dutch provinces following the assassination of his father, William the Silent (1625). A capable politician and military leader, Frederick led the way for Dutch independence. He ruled for more than a quarter of a century. Peace came to the Netherlands in 1648. The hymn was published in Adrian Valerius' Nederlandtsch Gedenckclanck (1626) and attributed to no author.
The English translation is by Theodore Baker (1851—1934). Baker was born in New York City and was literary editor for G. Schirmer Music Publishers (1892—1926). His Biographical Dictionary of Musicians (first published in 1900) became a standard reference work for musicologists. After his retirement he moved to Dresden, Germany, where he had been a student, remaining there until his death.

KREMSER, thought to be a folk tune from the Netherlands, was the original setting of the text. Eduard Kremser (1838—1914), a Viennese composer and choir director, rediscovered the hymn after two centuries of neglect. He arranged the tune for male voices and published it in 1877. In subsequent collections the tune has been named for him. Kremser was born and died in Vienna, Austria. He was also known for his instrumental works.