Gaining

How can i gain without my mum knowing?

Moving out's probably you're only option. That or just being honest. You're under her roof and if she flat out rummages through the food to check up on your diet and berates you for your weight, it seems fairly clear gaining isn't the best decision right now.

And while being honest will atleast making your standing clear and that it is your choice, She doesn't sound particularly like the understanding type. And she isn't obligated to keep you in her home either so she may threaten you with that standpoint.

If you want to gain it sounds like you're going to have to stand on your own two feet in this case. As gaining under her roof is going to be downright impossible.
6 years