My mom, 47, is overweight but not obese. She said that she woke up in the mid of night gasping for air. She fell asleepagain and woke up the same way again. Why could this happen?

May be obstructive s. Maybe obstructive sleep apnea. You can talk to your family doctor and get a sleep study to evaluate and confirm this diagnosis.
Reflux/sleep apnea. I would consider either nocturnal acid reflux or sleep apnea as the most likely problems. With sleep apnea the gasping is a sign of more serious problem and there are likely to be other symptoms like excessive fatigue and daytime somnolence.
Sleep apnea. She might have sleep apnea, this is associated with obesity but not in all cases. See http://goo.Gl/zwbqf.