I would have her heart checked asap. If nothing is causing the gaging, then it could be a sign of a heart problem. I took my 10yr old poodle to the vet thinking she had a cold. She has developed a grade 4 heart murmer. I also had a mini aussie I purchased for breeding. We figured she had a collapsed trachea. After doing her x-rays for OFA, we found out she had a bad heart. We went to a specialist, and she had faulty heart valves.
Gaging for no reason can be a big problem. I would really suggest haveing a vet listen to her heart, and make sure there isn't something going on.