C#

for( int i = 0; i < dataGridView1.Rows.Count; i++ )
{
    CheckBox ctl = (CheckBox)dataGridView1.Rows[i].FindControl( "CheckBoxControlName" );
    if( ctl != null )
    {
        ctl.Checked = ( i == e.RowIndex );
        ctl.Enabled = !( i == e.RowIndex );
    }
}

private void dataGridView1_CellContentClick(object sender, DataGridViewCellEventArgs e)
        {
            for (int i = 0; i < dataGridView1.Rows.Count; i++)
            {
                if (i == e.RowIndex)
                {
                    dataGridView1.Rows[i].Cells[0].Value = true;
                    dataGridView1.Rows[i].Cells[0].ReadOnly = true;
                }
                else
                {
                    dataGridView1.Rows[i].Cells[0].Value = false;
                    dataGridView1.Rows[i].Cells[0].ReadOnly = false;
                }
            }
        }

public partial class Form1: Form
    {
       internal static string textboxdata = string.Emtpy;
        public Form1()
        {
            InitializeComponent();
        }

 private void btnEkle_Click(object sender, EventArgs e)
 {
 textboxdata  =  txtGsm1.Text ;

}
}

txtGsm1.Text = Form1.textboxdata ;

1) Create a totally new variable, unconnected to any previous versions.

2) Access the correct old version of the variable that you had kicking around somewhere else in your program, but not to worry, the compiler will sort out which one your mean to use when it gets to it.

private void btnRehber_Click(object sender, EventArgs e)
   {
   Form2 rhbForm = new Form2();
   rhbForm.txtYetkili_isim.Text = txtYetkili_adi.Text;
   rhbForm.ShowDialog();
   txtGsm1.Text = rhbForm.txtGsm1.Text;
   txtGsm2.Text = rhbForm.txtGsm2.Text;
   txtNumara.Text = rhbForm.txtNumara.Text;
   }

public class Form2 : Form
   {
   ...
   public string Gsm1
      {
      get { return txtGsm1.Text; }
      } 
   ...
   }

private void btnRehber_Click(object sender, EventArgs e)
   {
   Form2 rhbForm = new Form2();
   rhbForm.txtYetkili_isim.Text = txtYetkili_adi.Text;
   rhbForm.ShowDialog();
   txtGsm1.Text = rhbForm.Gsm1;
   txtGsm2.Text = rhbForm.Gsm2;
   txtNumara.Text = rhbForm.Numara;
   }

private void btnRehber_Click(object sender, EventArgs e)
{
  using (Form2 rhbForm = new Form2()) // Please rename Form2.
  {
    // It's good practice to Dispose of forms. By wrapping
    // this in a using block, the form will be disposed of
    // automatically.
    rhbForm.txtYetkili_isim.Text = txtYetkili_adi.Text;
    if (rhbForm.ShowDialog() == DialogResult.OK)
    {
      // Only update the fields if the user clicked OK.
      txtGsm1.Text = rhbForm.Gsm1;
      txtGsm2.Text = rhbForm.Gsm2;
      frmFirma.txtNumara.Text = rhbForm.Numara;
    }
  }
}