I am also guessing it is GCF. I don't know about what he's doing on the left hand side though - I am not familiar enough with the stadella bass system to be able to decide based on hand placement. But it looks to me like he isn't even playing the left hand at all - it's a little hard to be sure.
My guess is based on the fact that the song is in D major, and he is playing it on the "pull" on the outer row and the middle row. You can see that quite clearly. D happens to be the "pull" key of G, which is the outer row of a GCF. So there you have it.