When I first heard of GSS I thought it was the holy grail of layout systems for app (not document) layout. On the surface, and as you mention, with simple examples it is deceptively alluring. However it quickly becomes apparent that the issues with regard to bugs and broken/cyclical dependencies far outweigh any benefits the system brings.

Maybe a hybrid approach would be best? Constraint-based for the broad strokes and then regular box-based layout within these blocks.