Fruit if she'll eat it....or very diluted juices; prune, etc. so that the constipation eases and she doesn't get dehydrated. You can make smoothies using a variety of things: peanut butter, fruits and veggies, yogurt, milk, etc. Look up some recipes...and that should get some calories in her. She may do better with that type of thing since her gums are bothering her.
If she won't go for anything, try to keep fluids into her. Also, you can medicate prior to eating times to help ease any gum soreness.
HTH.