In cases where you want to claim Gluten Free on a label you will likely need to include gluten in the NIP - as 0 % content. (Please check this point with a labelling consultant if you are not sure).
Zubi has chosen to use mg as the unit for gluten as it's seems more appropriate for this nutrient which sis declared as mg/kg on lab test reports.
When adding an ingredient to Zubi you will see gluten as an option - this is in place for the cases where users want to calculate the gluten contact in their product.
If you this ingredient is gluten free leave the gluten nutrient field as 0.00 mg.
When you open the NIP tab for the product you will see that gluten is an option in the Nutrition table. It will be unticked by default. This means it will not display on the NIP on the label.
If you tick the Display? box in the nutrition table then gluten will show on the NIP on the label.