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.