Preheat Oven: Preheat your oven to 350°F (175°C) and prepare your baking sheets with parchment paper.
Cream Butter and Sugar: In a large bowl, cream together the softened butter and brown sugar until light and fluffy.
Add Eggs and Vanilla: Beat in the eggs and vanilla extract until well combined.
Mix Dry Ingredients: In a separate bowl, whisk together the gluten-free all-purpose flour, gluten-free oatmeal, salt, cinnamon, and baking soda.
Combine Wet and Dry Ingredients: Gradually add the dry ingredients to the wet mixture, stirring until a dough forms.
Fold in Rhubarb: Gently fold in the finely diced rhubarb until evenly distributed throughout the dough.
Form Cookies: Drop tablespoon-sized portions of dough onto the prepared baking sheets, leaving space between each cookie.
Bake: Bake in the preheated oven for 15-18 minutes, or until the cookies are lightly browned around the edges.
Cool Cookies: Once baked, remove the cookies from the oven and allow them to cool on the baking sheets for 2 minutes before transferring them to wire racks to cool completely.