Gaining

Have you considered eating 'little and often'?

I.e. instead of having large meals you have relatively small portions/snacks which adds up over the day?

Keep a food diary of what you eat everyday and how many calories you are eating and drinking. Our bodies burn 2000-2500+ calories on average each day just to exist. Then you need a surplus of 3500 calories to gain just 1lb of fat.
When I started I had barely any appetite. I made sure to eat relatively normal and make sure I hit the daily calories and then used heavy cream to add the extra to gain. It took a week or so to start getting used to it and to not have to white knuckle it thru every excess calorie. But eventually it happens. Just like people who are trying to lose weight, gaining is a slow and steady situation. It's a lifestyle change. With a food diary you can make sure you are eating enough and then can also see how much extra you're taking in each day and can add those up to see how much you'll gain and how long it will take. There are plenty of calorie and weight gain calculators online and apps. Even when you start gaining it takes 5-10lbs of fat to start noticing it. It will happen. Just keep at it. Good luck!

If you struggle to eat extra calories then drinking them through the day may be easier for you. Doesn't have to be cream either - milkshakes/flavoured milk, sodas, energy drinks, juice, sweet teas etc. can all add extrta calories that add up over time.

To add onto this, you can find little ways to add onto the calorie count. Drink things with caloes - even if it's not calorie heavy. Juice, milk, coffee, tea - even adding flavor to your water adds calories. Add cheese, sauces, and condiments to your food as well.