The Ostermalm Saluhall has a great restaurant, Beirut Cafe, where we ate lunch. Very easy to get a gf meal here.
I did not see any gf options at the cafe at the Nobel Museum. I asked and got a great little traditional open-faced sandwich with gf bread and really tasty pickled herring and a little salad. I am so glad I asked!
The Old Haymarket food hall had many options. I just needed to ask. (Helsinki, Tallinn, Oslo and Bergen often label their menus with a key, showing all the allergens; didn’t see that quite as much in Stockholm.