Gluten is generally considered halal if it comes from plant sources such as wheat, barley, or rye. These grains are permissible and even encouraged by the Prophet Muhammad (PBUH) himself. For non-vegetarians, gluten derived from pork or non-halal slaughtered animals is considered haram. Therefore, gluten is halal if it comes from a plant source and haram if it comes from a non-halal animal source. And Allah knows best.

Sources:

• Halal Rule
• Pious Policies
• Mufti Says