What can make a gluten free sponge light and fluffy?

I have no trouble making lovely light lemon cake gluten free, and many other gluten free cakes too, I make them for a living. There is a substitute for gluten and it's called Xanthan Gum. You can buy this separately to add to your flour (about a teaspoonful for every 250g), or in a ready mixed flour blend, at least you can in the UK. A mixture of flours such as rice, potato starch, tapioca and maize/cornstarch works much better than a single flour when substituting for wheat flour. Each flour has its own unique characteristics and brings different properties to the cake. The combination of these, if got right, makes a light, moist and tasty cake, often better than wheat flour ones - this is feedback from my customers who are not coeliac!

There are recipes to be found on the web for making one's own flour blends if needed, but look for a recipe that contains Xanthan Gum and it will not turn out like a brick. And there's no need to cream the fat with the sugar and whisk the eggs separately etc. I just use an all-in-one everything in the mixer bowl and mix.

This is what I did when I make a Italian gluten free almond cake. Set aside five eggs to room temperature. Crack the eggs separate the egg whites from the yolks. Take the egg whites in bowl put in 1/4 teaspoon of baking soda whip for three minutes with a blender, add a couple dashes of sparkling water, and whip for a minute more then add the yolks and whip until blended. When the cake was baked it was fluffy and light I was thrilled . A easy gluten free cake made with just few ingredients, Almond flour, eggs, baking soda, sparkling water, almond extract of handful of slivered almonds. of course a oil sprayed cake pan.

You could try separating the eggs, whipping the egg whites, and folding them into the batter. Aside from that, perhaps some buttermilk and extra baking soda would help give additional lift.

The flour should be sifted, and a lighter flour should be used, as suggested by Daniel.

You could even replace some of the liquid in the recipe with club soda, but I'm uncertain of how well the batter will hold on to the extra air.

Try sifting the gluten free flour. Once at least then try your recipe. Perhaps, depending on the gluten free flour mix, you may need to sift twice.

Gluten is what traps the air bubbles during baking thus making a cake "fluffy". I don't really think there's a substitute...the best way I can think of is to mix some sort of lighter flour (cake flour, tapioca flour) in with the current recipe to decrease the density of the cake.

What sort of flour are you using currently?