Gabi Garcia would win in a grappling match with Megumi because she is too strong. She is able to muscle her way out of any situation. That is why Gabi is so dominant in BJJ. It doesn't matter how deep or tight the submission is. Gabi will get out.
What is interesting is that Megumi Fujii often trained with Josh Barnett. So maybe she knows some catch wrestling as well.
I would rather see Gabi fight Lei'd Tapa again. Assuming that Lei'd Tapa has been training since her loss to Gabi, it would be competitive. From watching their last fight, i think Gabi has a glass chin.