Yes, can be a challenge to mantain weight specially for those not overweight.
To avoid loosing weight, you have to keep energy levels more constantly. Eat healthy things that give you more energy and calories.
Eat avocado, add lots of extravirgin Olive oil, eat a lot of organic flax seed oil in healthy shakes, etc. For vitamins, don't go synthetic, get Superfood from Dr. Schulze or Garden of life.
You will certainly have a LOT of sugrar cravings (they tend to dimish with time)... so eat fruit... add nuts (brazil nuts are best=. Fruits are the key to mantain you in the right direction I believe, so you don't get back to grains. Eat more frequently and don't eat ENOURMOUS meals, to aid digestion.
The good thing is that there is a huge variety in fruits... so you don't get borred.
Clean you fruits and nuts with applec cider vinegar, let it a few minutes and then rinse (to remove mold and toxins).
When you eliminate all grains, I believe that fruit is not counterproductive for those with Candida (I'm seeing that on me). I eat chocolate occasionally and other candies with no effect now.
I'm underweight too, now, but I'm starting to gain weight by doing all of this, be constant, that's the key.
You lose weight because grains are the reason for having an overweight epidemic (no matter how GOOD they taste or their toxic chemicals make you feel); they cause/promote candida, cause diabetes, etc. Everything is backed up by Science (see Wai Genriiu's research, Mercola's, Cordain's, etc).
Don't go vegan, there's a lot of evidence that veganism is unnatural... it may help for some time but can't be good on the long run. We NEED animal foods (consuming toxic milk and Antibiotics .... well, that's ANOTHER story).
I "created" (maybe credits are not for me) this recipe based on Wai's advice:
-2 or 3 oranges
-2 Omega3 egg yolks
-2 Table spoons of Extra virgin olive oil