I have made the Chocolate Guinness Cake recipe quite a few times now and always a successful cake despite needing to adjust cooking times and temperatures for my oven. I did try a gluten free version a few weeks ago by substituting coconut flour. It was by comparison a disaster. The mixture thickened almost instantly as the Guiness, butter and cocoa mix hit it. I had to spatula it into the tin and press it together. At the same time and same oven I also made a regular, non gluten version and the batter was runny enough to pour. The end results were that the gluten free version was much lighter in colour, crumbly and broke up very easily. It was not horrendous to taste and the icing helped stick bits back together. The regular cake was dark moist and enjoyable as per usual. What would you recommend as a gluten free version if it is at all possible?
Nigella's Chocolate Guinness Cake from FEAST is a cake with a liquid batter and this gives a very moist cake. Often liquid batters can convert well to gluten free flours, which tend to absorb more liquid. However coconut flour is extremely absorbent and is not usually used as a direct one-for-one substitute for plain (all-purpose) flour. Hence this cake made with coconut flour would be quite dry and crumbly in comparison to one made with plain flour.
Unfortunately we are unable to advise on converting this cake to a gluten free version as we have not tested this option. But Nigella has a good selection of gluten free cakes on Nigella.com and you can now filter your recipe searches on the site to pick out gluten free options. A particularly popular gluten free chocolate cake is Nigella's Chocolate Olive Oil Cake.