Replace your code with this
protected void Page_Load(object sender, EventArgs e)
{
string connection = System.Configuration.ConfigurationManager.ConnectionStrings["itax1"].ConnectionString;
SqlConnection con = new SqlConnection(connection);
SqlDataAdapter da;
DataSet ds = new DataSet();
using(da = new SqlDataAdapter("Select * from pa_salary_master", con))
{
da.Fill(ds);
}
GridView1.DataSource = ds;
GridView1.DataBind();
}