We are surely in a minority in the population - I too have been having difficulty gaining weight after a long illness and the antibiotics I had to take.
Here are some strategies I used when I was on the meds, and constantly nauseated with little appetite. Over the past 12 months, I have successfully gained 5 pounds (I'm only 5'1" and trying to get to 110#)
These recommendations came from a nutritionist and a friend has lived with this issue for 7 years after having a feeding tube removed:
Frequent, small amounts of high-density good food - cashews, cheese, full-fat yogurt, healthy snack bars, hard boiled egg, lean turkey or chicken on whole-grain crackers...try for 200-250 calories every 2 hours.
Orgain Liquid Protein Drink - either from powder or single servings. If you are lactose intolerant, they have a plant-based one - or add the powder to any kind of soup or beverage.
Fruit-vegetable smoothies with added protein powder for lunch - loaded with whatever fruits and veg are in the frig or freezer - I make up 3-4 shakes at a time & store in refrigerator.
Carnation Instant Breakfast in my morning coffee, followed by breakfast of fruit & yogurt 2 hours later.
Sue