I had class 2 and had the surgery about 7 weeks ago (exactly the same type you're going to have), and my extraction gaps were not fully closed, I had maybe 1-2mm left on one side and 2-3mm on the other. I have self ligating braces that continued working so on one side my gap is now fully closed and on the other side there is maybe 2mm left. I don't think this was a big issue in my case, probably would have been better if the gaps were already closed but they did not make a big difference. However, any bigger gaps than that would have been an issue.
How fast are your teeth moving, does your ortho use power chains and are they tightened regularly? I had mine tightened every 3 weeks. To be very honest, if you have a 10mm gap now, I do not see how that will be almost closed by December. I think you should talk to the surgeon about this and ask how it will affect the plan.
Gaps aren't only a problem because the advancement will be less - they are a problem because if you have big gaps before surgery, how can they get the bite right? If they close the gaps later, you'll end up in class 2 again.