In a mixing bowl, combine the warm water and sugar. Sprinkle the instant yeast on top and let it sit for about 5-10 minutes until it becomes frothy.
In another bowl, whisk together the gluten-free flour blend, baking powder, and 1/2 teaspoon of salt until well combined.
Once the yeast mixture is ready, add it to the dry ingredients along with 2 tablespoons of olive oil. Mix until you have a smooth and thick dough.
Grease your baking sheet with 1 tablespoon of olive oil. Transfer the dough to the sheet and spread it out evenly. Cover with a clean kitchen towel and let it rise in a warm place for about 30-45 minutes, until slightly puffed.
While the dough is rising, preheat your oven to 425°F (220°C).
After the dough has risen, use your fingers to create dimples all over the surface. Drizzle the remaining olive oil over the top, and sprinkle with the remaining salt, chopped rosemary, and minced garlic (if using).
Place the baking sheet in the preheated oven and bake for 20-25 minutes, or until the focaccia is golden brown and sounds hollow when tapped.
Remove the focaccia from the oven and let it cool in the pan for a few minutes before transferring it to a wire rack. Slice and enjoy warm or at room temperature!