Ah goddamn you are one of those skinny guys that all you have going for you is a 6 pack. I bet you even flash it everytime someone asks if you lift, or run or if you eat dessert. You gotta get over that to put on weight. Nobody is saying to turn into a lard arse but gaining weight while keeping chiselled abs is really tough and takes a lot of experience, consistency and knowledge of what goes into you body. Not worth it IMO (although it is possible).
Dunno where you got that from? As long as you are getting 3-5 serves of vegetables in a day, don’t be afraid to eat a little dirty to move the scale. It tastes better and it is calorie dense. Kfc and service station chicken strips before work helped me immensely to grow my stomach when I started my gaining journey. Also high protein flavoured milk. And fatty fish. And so many potatoes!