Gas can also be caused by a latch issue i.e. a latch that isn't a complete seal where a baby is literally taking in air as they feed that then needs to come out. Or if a baby is bobbing on and off they make take in more air. So just review your latch too.
Also positioning makes a difference i.e. make sure the upper body isn't lower than the bottom half (which I did myself on a pillow until someone pointed it out
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Oversupply and overactive letdown are also common contributors (this can mean a higher proportion of thinner foremilk which contains much more lactose and contributes more to gas).
Infact overall the management of breastfeeding is much more a common cause of gas than a reaction to some food stuff entering the mother's bloodstream and then her milk.
There's a discussion here:
http://www.kellymom.com/babyconcerns/gassybaby.html"Formula-feeding moms never think it was what the cow ate the day she was milked, months ago!"
I think dairy is the most common culprit if there is a dietary issue - specifically either a reaction to a dairy protein or lactose.
