I'm not sure if, as a grain, rice is a gluten, but it's what Chinese people eat all the time and they eat more fiber than Westerners. The white and brown rices (brown rice is rice that's processed the same as white, just not bleached) in the store have low fiber content, but I have heard more natural rices (like wild rice) have more fiber; you might want to ask at a health food store. Also there are oats, provided they don't have gluten either. I have heard that the "steel cut oats" are much better for you than quick oats, more filling and have more fiber.
Basically all plant materials contain fiber--some contain more than others. about all you can do is up your vegetable intake seriously in order to compensate for the loss of wheat.
There are also chewable fiber pills (which I don't know, but they probably have wheat in them). Personally I found they CONSTIPATED me worse instead of less, so I don't set much stock in fiber; I have never noticed a difference in my bowel movements after increasing or decreasing my fiber, but maybe I just didn't make a big enough change. And some people do notice a real change, so I guess it depends on the person.