Gluten free cake

[Spireite]

Today I made the first part of my soon to be baptised Goddaughter's Christening cake.  Her Grandmother is a coeliac, so I got some Dove farm gluten free flour and used that for the fruit mixture.  I know these mixtures can be a little drier, so I added extra egg (about 2 and 1/2, instead of 2).  I've never done a tiered cake before so I was going to do a chocolate sponge for the bottom tier...I have my dowel rods ready! 
As the gluten free flour is plain do  I just add Bicarb/baking powder?? To make it usable for a sponge? I may have to try and do a trial run next week for the sponge (the baptism is in March), as my sponges are not as good as my fruits!

[Spireite]