When substituting gluten free all-purpose flour in baked foods, try substituting 1/2 volume of the original recipe all-purpose wheat flour with all-purpose GF flour plus 1/4 volume nut (like almond) flour. (You can also use weights). GF flours tend to absorb more liquids then wheat flour. Nut flour will absorb liquid more evenly and make the cake or cookie more moist. Also, letting the cake or cookies cool in the pan helps.