Advice for maintaining weight after cutting gluten?

I was underweight my whole life, and after finally achieving a weight that I was healthy with I noticed that I've already started to lose weight after three months of being on my diet. I feel like crying, but I don't want to lose hope.

If anyone else is going through something similar, are there any specific types of food/possible workout routines for building muscle that you've done to keep the weight on you?