Writer and Director Tim Blake Nelson made Dr. Miklos Nyiszli's memoirs "Auschwitz: A Doctor's Eyewitness Account" (1946) mandatory reading for the film's cast, along with Primo Levi's "The Drowned and the Saved" (1986) and Filip Müller's "Eyewitness Auschwitz" (1979).

The actual plans of the Auschwitz-Birkenau extermination camp and its crematoria were used to build the sets as historically accurate as possible. A 90 % scale reproduction of Auschwitz-Birkenau's crematoria II and III (in the film, counted as I and II) was built near the village of Giten, 45 minutes outside Sofia, Bulgaria. The result represents a fairly accurate depiction of a small part of the Auschwitz-Birkenau extermination camp.

Although Harvey Keitel played SS-Oberscharführer Erich Mußfeldt, he is of Jewish-Polish heritage.

Expert Andreas Kilian criticized the depiction of the Sonderkommando revolt in The Grey Zone (2001) as "exaggerated" in his 2014 article "Zum 70. Jahrestag eines Symbols des Widerstands". Earlier depictions of the revolt in the films Passenger (1963) and Triumph of the Spirit (1989) would be misleading, too, because f.e. the Sonderkommando prisoners had no guns and grenades to defend themselves, there was no explosion in a crematory, only a fire, and only one crematory was damaged. The cinematic depiction in Son of Saul (2015) is the most accurate yet.

This was the first feature film about the "Sonderkommando" at Auschwitz-Birkenau. The acclaimed Hungarian-language feature film Son of Saul (2015) became the second one. While both films dealt with the same historical events, including the revolt on October 7, 1944, they are told from different perspectives, in different styles, and highlight different themes. The Hungarian film also characterized the "Sonderkommando" less ambiguous. Both films are based in part on doctor Miklos Nyiszli's "Auschwitz: A Doctor's Eyewitness Account", but Son of Saul (2015) used additional historical accounts, especially "The Scrolls of Auschwitz", a.k.a. "Voices from beneath the Ashes" (edited by Ber Mark), a collection of secretly written and hidden testimonies by members of the "Sonderkommando".

Director of Photography Russell Lee Fine shot most of this film with a hand-held camera. ''A film like Schindler's List (1993) does this beautiful photography. We're not going to beat that quality. So we've given it an intentionally rough, hand-held look; made the images less romantic and less heroic. We want it to feel like you're there.''

The film's title is inspired by the second chapter of the essay collection "The Drowned and the Saved" by Holocaust survivor Primo Levi, first published in 1986. The second chapter is called "The Gray" (spelling correct) and deals in an analytical way with the subject of the "Sonderkommandos". Since Levi was never part of a "Sonderkommando" in Auschwitz, and never met one, his knowledge was mostly based on the critical description of Miklos Nyiszli in "Auschwitz: A Doctor's Eyewitness Account". That book is the main historical source of The Grey Zone (2001), too.

The film is included on Roger Ebert's "Great Movies" list.

Jeff Danna is the credited composer, but there is no score in the actual film. He only wrote music for the title and end credit sequences. He also arranged a classical piece used as source music.

This film had it's debut screening at the 2002 Woodstock Film Festival in Woodstock NY. Director Tim Blake Nelson hosted the screening and answered questions from the audience after the film.

Though she is a central character, the little girl does not speak during the entire movie. She only "narrates" the poetic closing lines (using "voice-over") after her death in the film.