Preheat oven to 350°F (180°C). Grease a 9" x 5" loaf pan and line with parchment paper. Zest and juice the lemons, you'll need 1 tablespoon of lemon zest and 4 tablespoons of lemon juice in total.
In large bowl, combine the gluten-free flour, granulated sugar, almond flour, baking powder, salt and lemon zest.
Separate the eggs, setting aside the egg yolks. Place the egg whites in a clean bowl, and using a hand mixer or stand mixer, beat the egg whites until firm peaks (the whites are holding their shape, but not too stiff, the tip of the peak curls over itself when the beater is lifted).
In another bowl, whisk together the greek yogurt, egg yolks, vegetable oil, milk, lemon juice and vanilla.
Add the yogurt mixture to the dry ingredients and stir until well combined.
Add a third of the egg whites to the mixture and stir until combined. Gently fold in the rest of the egg whites.
Pour the mixture into the prepared loaf pan.
Bake for about 50 minutes, or until an inserted toothpick comes out clean or with a few moist crumbs attached. If the loaf starts to brown too quickly while baking, loosely cover with aluminum foil. Let cool slightly in the pan, then transfer to a wire rack to cool completely.
To glaze loaf: Whisk together the powdered sugar, lemon juice and milk. Place the cooled loaf on a wire rack. Evenly pour the glaze over the loaf. Let the glaze set before slicing.