Brownies made with Ground almonds instead of flour are quick and easy to make and treat for everyone. Another alternative is a lemon drizzel cake made with polenta, I use a Nigella receipe I found on line and it is always popular. A flourless swiss roll, or sponge filled with cream is good as well.
Alternatively I just use the my favorite Delia all in one cake receipe with gluten free flour, but be careful with your baking powder, they do rise up and sink if you are over generous, works best with an extra egg and some added moisture like syrup, honey or marmalade in a loaf style tin.