No, cabbage is not toxic to buns. Yes, it CAN cause gas in some buns, but it doesn't in all.
It's the same as with people, some tolerate it without problems, some don't. You won't know untill you try.
Mine are ok with it, no gas problems at all (but then, I've yet to find anything that does...) except for Hazel who's poo can get a bit runny if she gets too much of it in one go.
Just as with all foods, try a small amount and see what happens.