Breastfeeding and Supplementing

Sadie
So I have been breastfeeding and baby latches like a champ since day one. She is a slow eater but eats well. I make plenty of milk and can pump between feedings and get 4+ ounces. However, when I got re-admitted to the hospital for postpartum preeclampsia and was on a strict fluid restriction, my daughters weight took a hit.  My daughters doctor said it might be good to give a meal a day of formula a day to help her catch up. She will drink the formula no problem but gets gassy. 
My first born was formula fed after 6 weeks and I didn't do supplementing so this is new for me. I feel bad giving something other then breast milk but I want to make sure baby is growing. Any words of wisdom on supplementing and helping with grassiness? 
Thank you