In cereals, gluten is formed in the grains and is only found there. When the grasses are harvested, gluten has not yet developed and they are accordingly gluten-free.
Is wheat grass gluten-free?

Living with Celiac Disease
In cereals, gluten is formed in the grains and is only found there. When the grasses are harvested, gluten has not yet developed and they are accordingly gluten-free.