I am hiv+ and under treatment last 1 year but I gain 15 kg what kind of diet should I follow with HIV treatment not to gain more weight please advice me?

Diet. Some weight gain comes from the side effects of the medications you're on. Generic advice would be to cut out foods that deliver high calories such as red meat, carb loaded foods - pasta, breads, sweets, whole milk dairy products, etc. Veggies are 0 calories, fruits contain healthier types of sugar. Use natural sweeteners such as agava, stevia or xylitol. I would refer you to a dietitian too.