Please go through below code, see if it helps. Not sure though if it is your requirement.
protected void Button1_Click(object sender, EventArgs e)
{
foreach (GridViewRow di in GridView1.Rows)
{
CheckBox chkBx = (CheckBox)di.FindControl("chkSelect");
if (chkBx != null && chkBx.Checked)
{
}
}
}