I also have no idea what big outer reaps are. How I would defend against the granby is to stop their momentum as they finish the roll. Catch their body. Block their hips. Elevate your body so they finish under you. I would just say you have to react fast regardless of what you do. Here's a video of my failed granby attempt. See if you can pick up on what the other guy did. Or maybe it's just what I did wrong. Either way it's 1:06 into the video.