I hate to say it, but I'd be willing to bet that gluten-free udon is about as practical as gluten-free seitan - the gluten is precisely what gives udon noodles the texture that makes them so special.
Having said that, I've bought frozen udon noodles that had tapioca starch as an ingredient in addition to wheat flour, and those were some of the best udon noodles I've eaten.
The other catch is that making udon noodles at home is hard - traditional recipes call for putting the very tough, springy dough in a plastic bag and stomping on it for a few minutes, as the only practical way of kneading it.
This site describes how to make udon noodles with tapioca - you could try replacing its flour with the gluten-free blend described here. I would also consider adding xanthan gum. If you're feeling adventurous, This e-book (not free, but looks pretty solid) discusses a wide variety of gluten-free flours and their properties, and might help you choose flours to use with the properties you want.
If gluten-free udon turns out to be impractical, I recommend soba noodles as an alternative. Those can much more easily be made gluten-free (although most commercially made ones available in the US do have wheat simply because it's cheap and bland) and can usually be used in the same dishes as udon. They have their own distinct texture, very different from udon, and a deliciously hearty flavor.
I thought that crepes (and tasty pancakes) were traditionally made with buckwheat flour. Crepes in particular because buckwheat makes a finer flour. So, traditionally made crepes would be gluten free.
Best Answer
I have no trouble making lovely light lemon cake gluten free, and many other gluten free cakes too, I make them for a living. There is a substitute for gluten and it's called Xanthan Gum. You can buy this separately to add to your flour (about a teaspoonful for every 250g), or in a ready mixed flour blend, at least you can in the UK. A mixture of flours such as rice, potato starch, tapioca and maize/cornstarch works much better than a single flour when substituting for wheat flour. Each flour has its own unique characteristics and brings different properties to the cake. The combination of these, if got right, makes a light, moist and tasty cake, often better than wheat flour ones - this is feedback from my customers who are not coeliac!
There are recipes to be found on the web for making one's own flour blends if needed, but look for a recipe that contains Xanthan Gum and it will not turn out like a brick. And there's no need to cream the fat with the sugar and whisk the eggs separately etc. I just use an all-in-one everything in the mixer bowl and mix.