int count = gridView1.Rows.Count;

for(int i=0; i < count; i++)
{
  GridViewRow row = gridView1.Rows[i];
  CheckBox cb = row.FindControl("CheckBoxID") as CheckBox;
  //Check if CheckBox is checked
  if(cb != null && cb.Checked)
  {
    // Logic here.
  }
}