Well, there are many that believe that inability to gain weight means that you are not absorbing carbohydrates properly. It would seem to me that candida could surely do that. Basically what I am saying is that candida could be causing you to be so thin.
The diet is tricky. I intitally lost weight while on a very strict diet. I have since added a few things that keep weight on me. As a mtter of fact I have gained a few pounds most likely due to exhaustion from die off .
I am not sure what your idea of a candida diet is, everyone has different ideas. But mainly you would want to eat antibiotic/hormone free meats and lots of vegetables. You could adjust this for your weight. For instance, added some starchy vegetables and beans which will add weight. Add buckwheat to your diet. You can make pancakes from the flour or biscuits. It does not aggravate candida at all. I have tested it on myself with a blood glucose meter and it did not raise my blood Sugar anymore than a salad did.
Also try beans and nuts. Nuts such as almonds and walnuts are okay if they ar fresh and raw. Also sunflower seeds and such, all contain fat and carbs along with protein.
The best thing you could do is go on a candida diet and bowel cleanse. If I were you I would probably Bowel Cleanse for a months first to see if that does not improve your absorbtion of foods and help you a little with weight. Then start adding antifungals.
Also, get checked for thyroid and diabetes, both can cause candida and weight loss and should be ruled out before you undertake a candida protocol. Treating for candida if you have thyroid issues or diabetes is fruitless.