I was 100lbs in 11-12th grade, thats when i started seriously working out and started powerlifting. I came out of highschool 160lbs If I estimate I gained all that weight in a year UNHEALTHY probably but weight was weight to me back then. I had someone actually push me to gain weight because I couldn't push myself too soo I have someone to thank. I literally would drink a gallon of milk throughout the whole day I would be shitting my brains out all day. I would skip school to go take a shit at a nearby college because their bathrooms were cleaner lol. When I was gaining weight I realized I would be skipping the most important meal of the day BREAKFAST I would NEVER eat breakfast, eating 3 MEALS A DAY makes a big difference. Before gaining weight I would only be eating 1-2 small meals a day. OHH AND I USED TO BRING A JAR OF PEANUT BUTTER WITH ME TO SCHOOL <--- everytime i started to get hungry take a scoop of peanut butter and eat it
i hope you understood my story lol its a bit confusing but yeah this generally summarizes my weight gaining experience