His parents were Gustav Neckel (1844–1923), an industrialist and businessman, and Amanda, née Paetow (1854–1914).
After completing his Abitur in Wismar in 1896, Neckel studied German philology at Munich (1896–1897), Leipzig (1897–1898) and Berlin (1898–1902), from where he received his doctorate in 1900 under Andreas Heusler. He then worked as a teacher until completing his Habilitation and becoming a lecturer at the University of Breslau in 1909.
Beginning in 1911, he was Professor of Old Norse at Heidelberg University, then in 1919–1920 at Berlin. From summer semester 1920 until 1935 he succeeded Heusler as Professor of Germanic Studies, with emphasis on the Scandinavian languages. From 1935 to 1937 he was founding Head of the Old Norse Division of the Department of Germanic Studies at the University of Göttingen, then from 1937 to 1940 Professor of Germanic Philology at Berlin, where he was, however, unable to work due to illness; he had a "nervous condition" from which he had barely recovered when he died suddenly of a pulmonary infection.
Neckel's career was disturbed by conflict with Bernhard Kummer and an accusation that he had seduced a student, which led to his being forced to leave Berlin and move to Göttingen; the chair, the most prominent in the field, came with him and a new division was created for him within the Göttingen Department of Germanic Studies.
Neckel resisted the politicisation of his department at Berlin and was open-minded on race and its relevance to his discipline; nevertheless, the increasingly völkisch point of view in his writings, his initial support for Kummer and Herman Wirth and his advocacy of the autochthonous theory of the origin of the runes have led some to see a marked decline in the calibre of his scholarly work beginning in the mid-1920s. His former teacher Heusler wrote repeatedly to his friend Wilhelm Ranisch that he seemed "no longer entirely sane" and that he seemed to have developed "an unhealthy ambition, not to say megalomania".
Neckel's research focused on early Germanic studies and Old Norse. He published the standard German edition of the Elder Edda. Continuing the approach of Jacob Grimm and of Heusler, he saw all Germanic sources, regardless of period or geographic location, as contributing to the picture of a unified Germanic culture. This culture he believed ethically superior to the medieval Christianity which overtook it, particularly in its respect for women. Heusler and others have considered conflict between ideologues within the Nazi regime, specifically between the Amt Rosenberg, the Ministry and the Ahnenerbe, at least partly to blame for his banishment to Göttingen.
- Walhall. Studien über germanischen Jenseitsglauben. Dortmund: Ruhfus, 1913.
- Die Überlieferungen vom Gotte Balder. Dortmund: Ruhfus, 1920.
- Liebe und Ehe bei den vorchristlichen Germanen. Leipzig: Teubner, 1932.
- (Ed.) Edda. Die Lieder des Codex Regius nebst verwandten Denkmälern. Volume 1: Text. Heidelberg: Winter, 1914. Volume 2: Kommentierendes Glossar. 1927. (Revised editions ed. Hans Kuhn)
- Gustav Neckel, Vom Germanentum: Ausgewählte Aufsätze und Vorträge, ed. W. Heydenreich and H.M. Neckel, Leipzig: Harrassowitz, 1944, OCLC 185170177, p. xiii (in German)
- Julia Zernack, "Neckel, Gustav (Karl Paul Christoph)", Reallexikon der germanischen Altertumskunde 2nd ed. ed. Heinrich Beck, Dieter Geuenich and Heiko Steuer, Berlin: de Gruyter, 2002, ISBN 3-11-017272-0, Volume 21, pp. 47-49, p. 47 (in German)
- Bernard Mees, The Science of the Swastika, Budapest/New York: Central European University Press, 2008, 978-963-9776-18-0, p. 178.
- Julia Zernack, "'Wenn es sein muß, mit Härte'—Die Zwangsversetzung des Nordisten Gustav Neckel 1935 und die 'Germanenkunde im Kulturkampf'" in Germanistik und Politik in der Zeit des Nationalsozialismus. Zwei Fallstudien: Hermann Schneider und Gustav Neckel, ed. Klaus von See and Julia Zernack, Frankfurter Beiträge zur Germanistik 42, Heidelberg: Winter, 2004, ISBN 3-8253-5022-3, pp. 113–208, p. 151 (in German)
- The circumstances are still obscure and described by his friend Wilhelm Heydenreich simply as his having been Opfer einer gegen ihn gerichteten feindlichen Strömung, der er in seinem überarbeiteten Zustand nicht gewachsen war (Vom Germanentum, p. xxii - rendered by Mees, p. 179 as "victim of a hostile current directed against him that he was not a match for given his overworked condition"). Mees regards the allegations as a pretext. See also Fritz Heinrich, "Bernhard Kummer (1897–1962): The Study of Religions Between Religious Devotion for the Ancient Germans, Political Agitation, and Academic Habitus" in The Study of Religion Under the Impact of Fascism, ed. Horst Junginger, Leiden/Boston: Brill, 2008, ISBN 978-90-04-16326-3, pp. 229–62, p. 251 and notes 93, 94.
- Fritz Paul, Zur Geschichte der Skandinavistik an der Georg-August-Universität Göttingen: Eine vorläufige Skizze, Skandinavisches Seminar, Georg-August-Universität Göttingen, 1985, retrieved 12 October 2010 (in German)
- Marie-Luise Bott, "'Deutsche Slavistik' in Berlin? Zum Slavischen Institut der Friedrich-Wilhelms-Universität 1933–1945" in Die Berliner Universität in der NS-Zeit, Volume 2 Fachbereiche und Fakultäten, ed. Rüdiger vom Bruch with Rebecca Schaarschmidt, Stuttgart: Steiner, 2005, ISBN 3-515-08658-7, p. 277, note 6 (in German)
- Heinrich, p. 249 and note 86.
- See for example Mees, p. 175.
- Klaus Düwel and Heinrich Beck, ed., Andreas Heusler an Wilhelm Ranisch: Briefe aus den Jahren 1890–1940, Basel: Helbing & Lichtenhahn, 1989, ISBN 3-7190-1022-8, cited in Fritz Paul: nicht mehr im vollen Besitz seiner Geisteskräfte; krankhaften Ehrgeiz, um nicht zu sagen: Größenwahn. According to Mees, pp. 175-76, Heusler "had come to the conclusion by 1933 that Neckel had simply gone mad".
- Reallexikon pp. 47-48, quoting: Die germanische Gesellschaft von Fürsten, Bauern und Sklaven, die wir aus den Sagas so genau kennenlernen [...], sie ist die germanische Gesellschaft der Zeit Attilas, der Zeit des Arminius und schon früherer Zeiten, die überall wesentlich dasselbe Gesicht zeigte. - "The Germanic society of lords, farmers and slaves which we come to know so well from the sagas . . . , that is the Germanic society of the time of Attila, the time of Arminius and even earlier times, which everywhere evinced essentially the same features".
- Zernack, "'Wenn es sein muß, mit Härte'", p. 178.
- Julia Zernack, "Gustav Karl Paul Christoph Neckel". In Internationales Germanistenlexikon 1800–1950, ed. Christoph König. 3 vols. Berlin: de Gruyter, 2003. ISBN 3-11-015485-4. Volume 2, pp. 1311–12.
- Germanistik und Politik in der Zeit des Nationalsozialismus. Zwei Fallstudien: Hermann Schneider und Gustav Neckel. Ed. Klaus von See and Julia Zernack. Frankfurter Beiträge zur Germanistik 42. Heidelberg: Winter, 2004. ISBN 3-8253-5022-3.