Make sure to use an all-purpose shop-bought gluten-free flour blend (or a homemade gluten-free flour blend), as opposed to just using any type of gluten-free flour you can find (such as almond flour, coconut flour, buckwheat flour etc).
Pour the syrup over the cake as soon as it’s out of the oven. The heat will help the sponge soak up all the flavour and keeps it extra moist. It may seem like a lot of liquid at the time, but it will all be absorbed into the sponge.
For optimum lemon flavour, I would recommend using fresh lemons (not ones that you've had sitting in your fruit bowl for weeks!) and organic ones if possible.
You have to be careful when baking with lemon juice as the acidity and lemon flavour can really vary depending on the lemons you use.
I would recommend tasting the lemon juice you're using and adjusting the amount of lemon juice you use accordingly.
For the lemon zest, make sure you use unwaxed lemons.