how to increment textboxvalue

You should first try to convert the TextBox.Text property from its own string type to int type:

int gpno;
if (int32.TryParse(txtbox.Text, out gpno) == false)
{
  // handle error: Text content does NOT represent a valid int
}
else
{ // succesful parse, here you may increment the value.
  gpno++;
}

yes you can

gpno = Convert.Toint32(txtbox1.text)+1;

If the textbox can contain only numerical values :

int value = int.Parse(textBox1.Text) + 1;

else use :

if (int.TryParse(textBox1.Text, out value))
            {
                value += 1;
            }

First get the textbox value in integer variable and than add 1 to it..

int txtboxText = Convert.ToInt32(txt1.Text);
int result = txtboxText + 1;

OR U can do this in a single line.

int result = Convert.ToInt32(txtbox.text) + 1;

and to restrict user to enter only numeric values to textbox You can use Filtered textbox extender.

int a = Convert.ToInt32(txtbox1.Text)+1;
gpno = a;