baking

I’ve been a cealiac for about 18 months now, and still am not doing well with baking.
Yesterday I baked a banana cake but into muffin cases instead of one large cake. The receipe was for normal SR flour which I substituted for White Wings Gluten-free SR flour. 1 1/2 cups of SR flour required. I used I cup gluten-free SR flour and 1/2 cup almond meal. The mixture was light and fluffy like – very light. Looked great going into the oven but didn’t work. Tasted great but sunk in the middle. I can’t work out what to do to get it right. This has also happened when I have used gluten-free SR flour instead of normal wheat flour in a cooking recipe. I don’t know what to do/use. I’d really like to make some nice cakes.
Anyone have some advice?