I haven't found masses of GF products in Auchan supermarket, and what there is is mostly to be found in the relevant food section or the organic/healthfoods section rather than in a dedicated Free From section. But I can highly recommend the Auchan own-brand baked goods. Vastly better than most of the GF baked goods I have tried in the UK. Especially the Payes Multicereal Rustico Multicereals bread and their own-brand pizza bases. The pizza bases come in twos, but we only made pizza once so I've been cutting up and toasting the other pizza base to have with my dippy egg - very good!
Another find in the bakery section (Portugal) has been the Almendrados cookies. One of which I am dipping in my tea as I post 😋
Anybody else have Portugal/Spain tips?