Change OnRowDeleting event code with below.

protected void OnRowDeleting(object sender, GridViewDeleteEventArgs e)
{
    int index = Convert.ToInt32(e.RowIndex);
    int Id = Convert.ToInt32(gvFiles.DataKeys[e.RowIndex].Values[0]);
    string constr = ConfigurationManager.ConnectionStrings["kankonConnectionString"].ConnectionString;
    using (SqlConnection con = new SqlConnection(constr))
    {
        using (SqlCommand cmd = new SqlCommand("vacation"))
        {
            cmd.CommandType = CommandType.StoredProcedure;
            cmd.Parameters.AddWithValue("@Action", "DELETE");
            cmd.Parameters.AddWithValue("@Id", Id);
            cmd.Connection = con;
            con.Open();
            cmd.ExecuteNonQuery();
            con.Close();
        }
    }
    BindGridView();
}