Hi Elsee and welcome. Have you been offered any dietary advice from your GP? It might be helpful to speak with your GP or a dietician to understand what your general calorie intake is and how you achieve above this to gain weight?
You have mentioned exercise, are you making any changes in your diet at the moment? They may be others here who have the similar dilemma of gaining weight, whilst avoiding raising blood glucose levels too high.
You may need to consider things that are high in fat, protein and calories, particularly unsaturated fats like oils, nuts, avocado.... Or moving to full fat dairy products for example? This all depends on what you like to eat of course!
But first, it is good to get an understanding of what the cause is for your weight loss, and what the best option will be for you in terms of healthy and safe weight gain.