Any recommendation for a gluten free grain that can be used instead of the pasta?

To follow up on gluten free pasta - there are bean based pastas on the market as well.

While I have no dietary restrictions myself, I have tried a couple of gluten-free pastas and one was actually decent. I think it was made of artichoke flour. Based on that experience, I would not hesitate to recommend using a tasty gluten-free pasta as a replacement.

Other alternatives would be dumplings based on potato or corn. If you replace the pasta with dumplings, you will likely need to adjust the cooking procedure so the dumplings don't turn into mush.

+1 groats! i bet rice would also work well.

whole millet perhaps?

buckwheat (a grass, not a grain) in the form of kasha groats.