First off, I wonder what “too skinny” is. In his younger years, my father was 5’10" and weighed 135 lb. He functioned perfectly well, (got through dental school, and that’s pretty taxing!!) but just had a light frame. My sister inherited his frame: she was 5’3" and weighed around 95-100 lb. Not a trace of fat on that girl, but she was physically very active and healthy.
So it seems to me that we come in a variety of shapes, and that some of us are meant to be thin and some of us in the middle, and some of us obese. The most important thing is how you’re functioning, mentally and physically. Dietitians often try to put us into boxes, but our bodies are pretty stubborn about where they want to be weight-wise.
However, if you really want to gain weight, eat a lot of fat – McDonald’s cheeseburgers without the bun, sausages sliced into your scrambled eggs, fatty bacon, heavy whipping cream in your coffee, etc. One of 2 things will happen – 1) you’ll gain weight and your dietitian will be proud until you tell her how you did it, or 2) your weight will maintain right where it likes to be, in which case you can tell her you sincerely tried, and ate the highest-calorie foods you could get your gums around and it just didn’t work, whereupon she will have to admit it’s the CARBS that put weight on people. And you will have to decide what kind of eating plan works best for you and makes you most comfortable.