Very gassy w/ Good Start Gentle?

[MuntryMomma]

The LO we're fostering is on GS Gentle. Pretty much from the beginning, she has struggled with being really gassy, though not spitting up unless she eats too much/too fast. I'm wondering if I need to consider changing formula? We've also been dealing with some constipation and I can't help but think it's all tied to the formula. Her Dr. isn't convinced as of yet and has recommended we try a couple of different bottles (she's currently using Dr.Browns)

Any thoughts please?