Yes, Gods of the Arena is a prequel, which was necessitated by the fact that the star of Blood & Sand, Andy Whitfield, was fighting cancer and could not film a second season at the time. Although thought cancer-free for a brief period, his cancer has returned, and he will no longer be able to play the role of Spartacus. In the meantime, the prequel was filmed and is now airing, while they have recast the lead role for Blood & Sand, the second season of which they will start filming in the near future.
I was a huge Blood & Sand fan and loved Andy Whitfield. I cannot imagine his replacement being successful, but wish him the best. I tried to watch the first episode of Gods of the Arena. I was interrupted, but from what I saw, I thought Blood & Sand was superior. I will have to give it another chance.