Your code is messy so i cant modify it for you. The way to get the sum is to loop through the items in the grid and sum them.
for example
int ItemToSum = 0;
for (int i = 0; i < GridView1.Rows.Count; i++ )
{
ItemToSum += Convert.ToInt32( GridView1.Rows[i].Cells[2].Text.ToString());
}
label1.Text = ItemToSum.ToString();