I don't have a lot of ideas, but the pp had a good one. You might try engaging her in converstion, too, like mention what you like and ask what she likes. Tell her you' like to get to know her better.
You might also ask her if she has an appetite. If she doesn't, that should be addressed.
Also, does she like milkshakes? You could make a smoothie with fruit, yogurt, and sneak in some high calorie and high nutrient foods (nut butter comes to mind). Be sure to sweeten it to her satisfaction.
And when you find out what she likes to eat, try to sneak in some high calorie/high nutrient foods into those. You could ask about how to do that with the specific foods once you know what they are (you'll get lots of ideas on the main Nutrition and Good Eating forum).
Another thought is to give her enzymes. I've been reading a lot about them lately, and they help with all kind of things. They will bring a person toward their ideal weight. Basically, it helps a body use all of the nutrients in the food, so even if she doesn't eat very much, the food she does eat will do more for her. A good website to get started is http://www.enzymestuff.com/
. She discusses the different types of enzymes, so that you can make sure you're taking the right types for the way you eat.
Good luck, and enjoy your grandma!