If you're just under your calorie target, you can add high calorie snacks or toppings to gain weight. Do you like nuts and seeds? Avacados? Olive oil? Cheese?
You can buy or make lots of nice high fat nut snacks, roast them with your favourite spices if you like flavour or just a little salt if you prefer blander stuff. Snack on them throughout the day, and schedule times to have a portion of you're not naturally a snacker. Sprinkle them over salads or rice bowls or whatever.
Dip nice bread in olive oil and vinegar (with some zatar or other spice mixes if your like). Shallow fry things in olive oil - take slices of nutrious but calorie light vegetables, dip in a simple tempura batter of cornflour and water, then fry in olive oil. Crispy, fresh, delicious and lots of calories.
So you eat desert? Start having nice fruit with heavy cream poured over the top. Use cream in sauces or add to mashed potatoes, vegetable gratins, etc.