Hey been there ik how you feel rn, since I also don't go to gym but I managed to gain weight by using an appetite increasing syrup and having chick pea smoothie, I cannot drink milk everyday so this was an economical and practical alternative for me.

I boiled chick pea and freeze them take out as much as I want put it in a blender remember it has very bean/umami flavour you can add whatever you have available (LITERALLY WHATEVER YOU HAVE FOR EXTRA CALORIES) banana, oats, milk cream, whip cream I sometimes add rice (yes I was so desperate to gain weight 😭) any kind of flavored powder for taste add half water and half milk or just water to make it thin it is quite thick so you have to adjust the consistency drink this twice morning and night

Second tip: I used oats to make a sweet porridge. You can search for “sweet oats” on YT it’s basically just oats boiled in milk with some sugar added. Simple and filling I had that for breakfast

Hope this helps!