Preheat oven to 350°F. Grease and line a 9-inch springform pan with parchment paper.
In a wide saucepan, combine 1 cup granulated sugar and 1 cup water. Bring to a simmer over medium heat.
Add orange slices in a single layer and simmer gently for 15-20 minutes, turning occasionally, until translucent.
Transfer candied oranges to a wire rack to cool, reserving the syrup.
Arrange cooled candied orange slices on the bottom of the prepared pan.
In a large bowl, whisk eggs and 1 cup sugar until pale and slightly thickened, about 2 minutes.
Add olive oil or melted butter, vanilla extract, orange zest, and salt; whisk until smooth.
Fold in almond flour and gluten-free baking powder gently until combined.
Pour batter over the arranged orange slices and smooth the surface.
Bake for 45-50 minutes until golden brown and a toothpick inserted in the center comes out clean.
Let cake cool in the pan for 15 minutes, then invert onto a serving plate so candied oranges face upward.
Brush the top with reserved orange syrup for shine and flavor.
Garnish with sliced almonds and dust with powdered sugar if desired. Slice and serve.