I don't know if the problem is the change in diet - those aren't really gassy foods. I do agree that the most likely culprit is the teething. And like the other poster said, there's not much you can do but give your daughter some tylenol and cuddle with her. When my son teethes, I just give him constant doses of Tylenol or baby Motrin to take the edge off, and I plan it so he gets the doses about a half hour before I expect him to nap or go down for the night. Talk to your doctor about the right dosage because although it's still safe for a 6 month old, the bottles are only labeled for older babies. During the day, letting your daughter suck on cool, wet washcloths may help.
I think you're doing the right thing so far - your daughter is most likely not able to settle herself back down to sleep overnight because of the pain, so holding her and comforting her is what she needs to do it. I know how hard it is to start getting up again overnight when you've just gotten used to uniterrupted sleep! But when teething is involved, it's what she needs. I'd steer away from Ferberizing her just yet - if she is teething, she's probably in a lot of pain and letting her cry it out isn't fair and isn't going to help. Have you tried baby Anbesol? My son didn't like it but I hear from other moms that it helps.
My little guy started teething at 2 1/2 months. He's 15 months old now and working on his 2 year molars. He's a teething powerhouse. Take it from me, it's tough! Good luck, and it should all be over in a few weeks... until the next teeth come!