Hi there, thanks for your question!
The short answer is yes to both.
Huel is designed the give the body the recommended ratios of macro nutrients (carbs, fats and proteins) and micro nutrients (vitamins and minerals). These are what the body needs to function.
In terms of fat loss / gain it simplistically comes down to calories in (food and drink) minus from calories out (what your body uses).
If you want to gain weight, the rough rule of thumb is to consume 500 calories over what you body needs per day, while if you want to lose weight, try to consume 500 under you daily requirements of calories.
Again roughly, 3,500 calories is 1lb. of fat. Therefore, if you consume a deficit of 500 calories a day, you could conceivably lose 1lb. of fat per week and vice versa.
The amount of calories your body needs per day depends on your age, gender, current weight and activity level. Use an online calculator to find out yours, e.g.