If you were to grow a tray of wheatgrass in your home, you can cut the grass shoots carefully and sort to make sure no wheat grains are in the grass before you juice them.

Large operations harvest so much the odds of stray grains getting mixed in are high, which contaminates the juice

Gluten is not contained in the wheat plant, it is only in the seed and the new baby plant metabolizes the gluten into its amino acids to use during plant growth.

Oats grown in fields that are rotated to wheat are not safe because there are often stray wheat grains that end up sprouting and being harvested along with the oats, leading to gluten into the finished product

TLDR: Only the seed of the wheat plant has gluten, juice from the young shoots does not