Some vaginal discharge is normal. If you have had an evaluation done and no infectious agent or other reason for discharge is found then you can just attribute it to your normal physiology. You may want to increase your intake of yoghurt to maintain the normal environment in your vaginal vault. I do not think that the leucorrhoea is in any way responsible for your inability to gain weight.
I would suggest getting your thyroid and sugar checked. The general principles of weight gain/loss are based on calories in vs. calories out. If you increase your calorie intake by increasing calorie dense foods (fats, sweets) and do not exercise much, you will gain weight. Half your weight gain comes from the genetics of your family and the other half from your diet. If most people in your family are skinny, then you probably have a genetic predisposition to be thin.