01 - Preheat oven to 350°F. Grease an 8x8-inch square baking pan and line with parchment paper.
02 - Whisk together gluten-free flour, baking powder, baking soda, and salt in a medium bowl.
03 - Beat softened butter and granulated sugar in a large bowl until light and fluffy. Add eggs one at a time, then stir in vanilla extract.
04 - Mix in Greek yogurt until completely smooth and incorporated.
05 - Gradually fold the dry ingredients into the wet mixture until just combined. Avoid overmixing to maintain tender texture.
06 - Combine brown sugar and cinnamon in a small bowl for the swirl filling.
07 - Spread half the batter into the prepared pan. Sprinkle evenly with cinnamon-sugar mixture. Dollop remaining batter on top and gently spread to cover.
08 - Mix oats, brown sugar, melted butter, and cinnamon in a separate small bowl. Sprinkle evenly over the batter.
09 - Bake for 38-42 minutes until a toothpick inserted in the center comes out clean.
10 - Cool in the pan for 15 minutes, then lift out using parchment paper and cool completely on a wire rack. Slice into squares and serve.