I've never had a gruit before, and certainly never tried to brew one. However, I am planning to brew one this coming Friday. I have a few yeasts in the fridge and I am leaning towards Wyeast 1968. Any compelling reasons not to use that yeast?
Another option would be WLP568, Saison blend, but that might be too odd...
Another option would be WLP568, Saison blend, but that might be too odd...