I don't know officially but I think GOTA probably takes place in 79 BC. It depends how much time passes in GOTA, I don't think it could be more than a year. It seems likely that S1 started in 74 BC as we know that the S1 finale and all of S2 is set in 73 BC. Though it's possible that the very beginning of the show (with Spartacus fighting for the Romans) happened over a longer period of time. Interestingly we know certain dates in the show due to references to real festivals, for instance 1x03 takes place in August (for the Vulcanalia).