How do I return a list Product group by Category

public IQueryable<GroupModel> GetAll()
        {
            List<Category> listCategories = new List<Category>
            {
                new Category {Id = 1, CateName = "SmartPhone", Description = "aaaaaa"},
                new Category {Id = 2, CateName = "Laptop", Description = "bbbb"},
                new Category {Id = 3, CateName = "Desktop", Description = "ccccc"},
            };

            List<Product> listProducts = new List<Product>
            {
                new Product {Id = 1, ProdName = "Lumia 720", CategoryId = 1, ColorId = 2},
                new Product {Id = 2, ProdName = "PC HP", CategoryId = 3, ColorId = 1},
                new Product {Id = 3, ProdName = "PC Dell", CategoryId = 3, ColorId = 1},
                new Product {Id = 4, ProdName = "Laptop Lenovo", CategoryId = 2, ColorId = 2},
                new Product {Id = 5, ProdName = "Lumia 920", CategoryId = 1, ColorId = 2},
                new Product {Id = 6, ProdName = "Laptop Dell", CategoryId = 2, ColorId = 3},
                new Product {Id = 7, ProdName = "Laptop HP", CategoryId = 2, ColorId = 3},
                new Product {Id = 7, ProdName = "Lumia 1020", CategoryId = 1, ColorId = 1}
            };

            var query = (from p in listProducts
                         join co in listColors on p.ColorId equals co.ColorId
                         join c in listCategories on p.CategoryId equals c.Id into e
                         from j in e.DefaultIfEmpty()
                         
                         select new GroupModel
                         {
                             CategoryId = j.Id,
                             CategoryName = j.CateName,
                             ProductId = p.Id,
                             ProductName = p.ProdName,
                             ColorId = co.ColorId,
                             ColorName = co.ColorName

                         }).ToList().GroupBy(x => x.CategoryName);



            return query.AsQueryable();
        }