Preheat oven to 350F and prepare an 8x8in tin with parchment paper.
Combine gluten-free flour, almond flour, powdered sugar, cornstarch, salt, and cold, cubed butter in a large bowl or bowl of a stand mixer equipped with a paddle attachment. Mix at a medium speed until it resembles wet sand (should hold together when pressed but remain easily crumbled), about 3 minutes.
Press dough evenly into tin. Bake until lightly golden around the outside and matte/dry in appearance throughout, about 20-22 minutes. Remove from oven and egg wash before baking an additional 1-2 minutes. Remove from oven and set aside.
When crust is nearly done, mix egg, granulated sugar, cornstarch, ground ginger, and salt together in a large bowl. Once homogenous, add Meyer lemon juice and continue mixing until incorporated. Pour over baked crust.
Reduce oven temperature to 300F. Return tin to the oven for approximately 30 minutes, until it appears set when jiggled lightly.
Remove from oven and let cool to room temperature in the tin. Remove from the tin, cover, and refrigerate at least 3 hours (ideally overnight). For best results, cut into squares after refrigerating.
Keep refrigerated until ready to serve. Use a mesh sifter to dust with powdered sugar before serving.