Preheat oven to 350° Fahrenheit. Lightly grease 9x5 loaf pan with cooking spray or butter, then set pan aside. Place parchment paper on flat surface and set wire cooling rack on top. Set aside.
Add gluten-free flour, almond flour, baking powder, and salt to medium bowl. Whisk until dry ingredients are fully incorporated. Set aside.
In large bowl (or bowl of stand mixer), use mixer to cream softened butter and granulated sugar. Beat until mixture is pale yellow and fluffy, approximately 2 to 3 minutes.
Add eggs one-by-one to butter mixture and mix to incorporate, then add lemon zest, lemon juice, and vanilla extract. Mix until all ingredients are incorporated.
Slowly add dry ingredients to butter mixture, mixing constantly until all ingredients are just incorporated. Be careful not to overmix.
Transfer batter to greased loaf pan and spread into one even layer. Place loaf pan in oven and bake 55 to 65 minutes, or until knife or toothpick inserted into center of cake comes out completely clean or with very few crumbs.
Toward end of bake time, but before removing cake from oven, add sugar and lemon juice to small bowl. Whisk until sugar is fully dissolved and mixture has smooth consistency.
When cake is ready, remove pan from oven. Remove cake from pan immediately, running knife along edges to loosen if needed. Place warm cake on wire cooling rack over parchment paper and immediately drizzle lemon-sugar mixture over top of cake.
Let cake and drizzle cool completely. Drizzle will harden slightly as it cools. Garnish with lemon slices if desired, then slice cake and serve.