I sell on a farmers market and I have been making for a while a gluten free raspberry cake https://cookingforthesensitivegut.com/2016/02/03/gluten-free-raspberry-lime-yogurt-cake/. It's quite popular but I'm thinking as the weather is getting colder it is a bit of a summery cake. Any recommendations for a good recipe that maybe uses fruits for this season such as pears or blackberries? Or more spiced and cinnamon flavours 

lane cake is a traditional holiday christmas cake -- zillions of recipes for it online

Looking at your linked recipe it said blueberries could be sub'ed.  Most any fruit -especially berries -  I bet would work.   You are going to have to try something like this yourself because most if not all of us don't use that recipe &  we each have different taste buds - so what tastes good to me  might not go over so good w/your customers :) 

Another possible combination would be apple cinnamon.  I made one last week.   Wasn't bad:)  Of course it wasn't anything like you recipe so again, you will have to test it out.   And if the apple works I bet pears would.   I'd probably test it using both applesauce and fresh grated (fine chopped) apples.  Is cinnamon 'gut friendly"? 

With that base already having hazelnut's listed, poached pear or apples, or blackberries would be nice in it. Maybe change the zest to lemon or orange and add some spices.