The garment bag will count as your allotted carry-on bag. Whether Flight Attendants allow you to stow your garment bag in the scarce closets or not is a crAAp shoot and generally unlikely, particularly in coach, and I’d not count on it. Use the plastic sheeting that comes from the laundry protecting your dry cleaning, etc. to interleave with your suit so it won’t look weedy when you take it out.