I know the scene- Pullo asked "who?" then Vorenus mentioned Gracchi. And yes, it believable: Pullo is commoner, son of slave so his education is hardly anything to speak of, so he might be unaware of the politicians who lived century ago.

However it is near impossible, that he would not knew about Marius (or Sulla for that matter). Memory of Sulla's civil wars was still very alive especially because memory of Sulla-Marius wars was still very alive. It was a very traumatic experience for Romans And hardly anyone could escape references to the that period- Ceasar start his political career by restoring statues of Marius (Marius himself started his career by marrying Julia Caesaris). Sulla's son was still alive (he was killed soon after Thapsus) and so on.

Plebeian, especially not native Roman (Pullo is from Umbria), could fail to recognize name of Gracchi- he lived century ago and there was big events to overshadow him, but he could not avoid at least basic knowledge about Marius.