We gate checked 2 car seats and one large double jogging stroller in March when we flew Delta. Never check your car seats. They go through so much torture that they would have to be scrapped and you would have to get a new one. The airline does not consider them luggage and you are not charged extra for them. There is a place on the plane for "delicate" things, like wheelchairs, car seats, and strollers. They don't have to go on any conveyor belts or anything that way.
When we got to the gate we put a special tag on our seats and stroller that the gate attendant gave us and the airline personnel packed them on and off the plane for us. It took a few minutes to get the stroller after we de-planed, but it was no big deal. DH just took care of the seats and stroller while I took care of the two littles, worked out great.