This is how I would get visibility of the category Grids in the only linked model in the active view:
UIDocument uidoc = commandData.Application.ActiveUIDocument;
Document document = uidoc.Document;
FilteredElementCollector links = new FilteredElementCollector(document, document.ActiveView.Id).OfClass(typeof(RevitLinkInstance));
Document linkedDoc = links.Cast<RevitLinkInstance>().FirstOrDefault().GetLinkDocument();
Categories categories = linkedDoc.Settings.Categories;
Category grids = categories.get_Item(BuiltInCategory.OST_Grids);
bool visible - grids.get_Visible(document.ActiveView);
Is this what you want to do?
