All hail the benevolent politician: Gracchus' role in Gladiator is to act as the face of the Senate. Sure, there are other senators (Falco and Gaius, for example), but Gracchus seems to be the most respected.
He's a morally upright guy, and he's committed to ending the tyranny of Commodus' rule. However, he also knows how difficult it'll be to depose an emperor that's well liked by the citizenry—and Commodus has quite a fan base because he throws a good gladiatorial fight.
But Gracchus has a close relationship with Lucilla, and through her he's able to meet with Maximus and concoct a plan.
While Gracchus is initially reluctant to free Maximus and let him return with an army, Maximus makes him realize that the "time for half measures and talk is over" and that something must be done, however drastic.
Gracchus goes along with the plan, knowing that the fate of Rome depends on it: republican government must be restored. He's ultimately arrested for his actions, but Commodus is killed before Gracchus can be silenced forever.