Fellowes is also nominated for an Academy Award for his screenplay.
Akiva Goldsman, who won the Golden Globe in late January, won for best screenplay based on material previously produced or published for "A Beautiful Mind." The drama about schizophrenic math genius John Nash is based on the award-winning book by Sylvia Nassar. Like Fellowes, Goldsman is also nominated for an Oscar.
"A Beautiful Mind" and "Gosford Park" are also Academy Award nominees for best picture.
Several other Oscar-nominated screenplays, including Christopher Nolan for "Momento" and Todd Field for "In the Bedroom," weren't eligible for the writers guild awards because their films were not WGA signatories.
Terence Winter (writer and story) and Tim Van Patten (story) were honored by the WGA in the episodic drama category for the "Pine Barrens" episode of HBO's "The Sopranos."
Philip Rosenthal won the episodic comedy award for "Italy, Parts 1 & 2" of CBS' sitcom "Everybody Loves Raymond."
Other major winners at the ceremony, which took place at the Beverly Hilton Hotel in Beverly Hills and the Pierre Hotel in New York, included Loring Mandel for original long form for the HBO movie "Conspiracy" and Kirk Ellis for best adapted long form for the ABC miniseries "Anne Frank."
Previously announced honorary awards were presented to recipients at the L.A. ceremony, including Blake Edwards (the Screen Laurel Award), Glenn and Les Charles (the Paddy Chayefsky Laurel Award) and, posthumously, David Angell (the Valentine Davies Award).
Screenplay written directly for the screen: Julian Fellowes, "Gosford Park."
Screenplay based on material previously produced or published: Akiva Goldsman, "A Beautiful Mind," based on the book by Sylvia Nassar.
Original long form: Loring Mandel, "Conspiracy," HBO.
Adapted long form: Kirk Ellis, "Anne Frank," based on the book by Melissa Muller, ABC.
Episodic drama: Terence Winter (teleplay and story) and Tim Van Patten (story), "Pine Barrens" ("The Sopranos"), HBO.
Episodic comedy: Philip Rosenthal, "Italy, Parts I & 2" ("Everybody Loves Raymond"), CBS.
Comedy/variety--music, awards, tributes, specials, any length: written by Don Baer and George Stevens Jr., film sequences written by Sara Lukinson, Harry Miles Muheim, "The Kennedy Center Honors," CBS.
Comedy/variety (including talk) series: Mike Sweeney, Chris Albers, Ellen Barancik, Andy Blitz, Kevin Dorff, Jonathan Glaser, Michael Gordon, Brian Kiley, Michael Koman, Brian McCann, Guy Nicolucci, Conan O'Brien, Andrew Secunda, Robert Smigel, Brian Stack, Andrew Weinberg, "Late Night with Conan O'Brien," NBC.
Daytime serials: Agnes Nixon, Jean Passanante, Craig Carlson, Frederick Johnson, N. Gail Lawrence, Victor Miller, Juliet Law Packer, Addie Walsh, Mimi Leahey, Bettina F. Bradbury, Charlotte Gibson, David Hiltbrand, Janet Iacobuzio, Royal Miller, John Piroman, Rebecca Taylor, Neal Bell, "All My Children," ABC.
Children's script: Anna Sandor, "My Louisiana Sky," based on "My Louisiana Sky" by Kimberly Willis Holt, Showtime.
Documentary--current events: Lowell Bergman and Kenneth Levis, and Doug Hamilton and Oriana Zill, "Drug Wars, Part 2" ("Frontline"), PBS.
Documentary--other than current events: Rushmore DeNooyer, "Hitler's Lost Sub" ("Nova"), PBS. Tied with Barak Goodman, "Scottsboro, an American Tragedy" ("The American Experience"), PBS.
News--regularly scheduled, bulletin or breaking report: Jonathan Kaplan, "Wedding Disaster," CBS-TV (WBBM).
News--analysis, feature or commentary: Jonathan Kaplan, "The Cruelty Connection," CBS-TV (WBBM).
Radio documentary--any length: Wendy Zentz, "Eye on Death Row" ("Weekend Roundup"), CBS Radio Network.
News--regularly scheduled: Paul Farry, "The Recount," CBS Radio Network.
News--analysis, feature or commentary: Scott L. Anderson, "Preserving American Sound" ("Perspective"), ABC News Radio.
On-air promotion: Lori Sunshine, "NBC Promotions," NBC.