Wow... I am surprised you didn't lose weight when you went gluten-free for your baby! Most of us do lose some weight when we eliminate the wheat, barley and rye... maybe because our whole life-style of eating changes. I admit, though, the 3 of us in my family also had to eliminate dairy and that had a big effect, too.
At any rate, You can lose weight safely while nursing and eat a very nutritious diet! Many people lose weight while nursing without even meaning to, just because the baby is drinking up our extra calories.
As others have said, it is not your extra calories that provide high quality milk - it is plenty of water and good clean food... I'd go organic as much as possible. And keep up with exercising and being very active. A healthy diet often leads automatically to a healthier weight. Just be sure you are eating healthy and taking any supplements your doctor says because it is YOUR body you need to protect with the vitamins and minerals as much as it is your baby's. I was told, the baby will take what it needs in nutrients even if it is at the expense of your own bones and muscles. So that is what is important here - you have more than enough calories. So just make sure you have nutrients (which does not come from PB or crackers, etc) - not the calories.