Dreamworks SKG and Scott Free Productions
A movie like Gladiator, with an estimated $100 million dollar budget, doesn't get made without a whole lot of producers.
In fact, it's easier to determine who doesn't have a production credit than who does. It's a very murky discussion, and lots of different people—and at least a few companies—played a part.
The biggest player is Dreamworks SKG, a production company started in 1994 by Steven Spielberg (S), Jeffrey Katzenberg (K), and David Geffen (G).
But none of those three Hollywood bigwigs were really that involved with Gladiator.
Spielberg did play a small role, however. David Franzoni, who created the original story and wrote the first few drafts of the screenplay, had recently worked with Spielberg on the historical drama Amistad (1997).
He pitched the idea that would become Gladiator to Spielberg, who realized what a mega-blockbuster this film was destined to become. While the exact extent of Dreamworks' involvement in the production of Gladiator is unclear, the company certainly did distribute the film, and at least one prominent Dreamworks producer, executive producer Walter Frankes played a big role in the development of the film.
It was Parkes, along with fellow producer Douglas Wick, who took the idea to Ridley Scott, the director they'd singled out as the right guy for the job. Given Scott's interest in art, Parkes and Wick brought along a famous painting of a gladiator in order to pique Scott's interest in the film.
Needless to say, it worked. Scott signed right up.
Ridley Scott's a very hands-on sort of dude, and he contributed to the production of the film, even though he isn't credited as having done so. The production company that he and his late brother Tony Scott established, however—the aptly titled Scott Free Productions—is.
Scott Free Productions has been involved in all sorts of action-packed projects. Here's a short list: Hannibal (2001), Black Hawk Down (2001), Man on Fire (2004), and Kingdom of Heaven (2005).
And yes—all of those movies were directed by either Ridley or Tony Scott, but the company has handled movies for other directors, such as 2010's A-Team (directed by Joe Carnahan) and a whole bunch of TV shows and specials (such as 2013's Killing Lincoln and Killing Kennedy.)
In addition to Scott, Wick, Parkes, and Franzoni (who has a production credit on the film) there's also Parkes' wife, Laurie MacDonald, and, most importantly Branko Lustig (best known at the time for his work on Schindler's List (1993).
When a film wins a Best Picture Oscar, the award technically goes to the film's producers. When Gladiator won that most coveted of awards in 2001, it was Franzoni, Wick, and Lustig who received the award. (Source)
Based on that alone, it seems that those three individuals that did the most in terms of developing the actual film as it was released.