I've eaten at Kverneriet in Oslo at both Majorstuen and Solli, and also at their restaurant in Tønsberg.
I've always experienced helpful staff that's made me feel safe ordering burgers with GF rolls/bread that's is celiac safe and fries that are celiac safe. I've spoken to their management through my GF page on IG and they do have dedicated fries for fries/chips.
What they can not assure here is no cross-contamination at all as they kitchen also do make food containing gluten, but I've eaten here too many times to even count and never had any reactions.
I absolutely recommend this place for delicious burgers and fries, not to mention yummy dessert!
— Somewhat
— Gluten-free items are marked on the main menu
— Some
— Excellent