I don't see James Gunn cutting out any Robin in favor of Damian. He did like a tweet about Jason showing up and that's more confirmation than Batman and Robin's books being in an image and then him confirming that actually isn't a hint.
I think what's more likely is we skip to Tim as Robin and build up to Damian or just go fully New 52 with it. But I think skipping to Tim's Robin would be more effective.