Is there any way to use colspan with GridView.builder in Flutter?

Question

I'm using GridView.builder and looking for a way to add colspan for a specific child to take the full width. In the below example, I have a 2-column GridView which each column represents a product, but whenever user reaches to the end of the list, I want to show a loading indicator, which in this case it is shown in the first column on the left. I want the Center widget to take 2 columns space like colspan=2 and show the loading indicator in the center.

GridView.builder(
  itemCount: products.length + 1,
  gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(
    crossAxisCount: 2,
    crossAxisSpacing: 8,
    mainAxisSpacing: 8,
    childAspectRatio: MediaQuery.of(context).size.width /
                          (MediaQuery.of(context).size.height / 1.8),
    ),
  itemBuilder: (BuildContext context, int index) {
    if (index >= products.length) {
       return const Center(child: CircularProgressIndicator());
    }

    return ProductListItem(product: products[index]);
  },
),