The link you provided is for Windows Forms. It does not work like that.
Try the below,
CS
(GridView1.Rows[YOUR_ROW_INDEX].FindControl(YOUR_CONTROL_NAME) as TextBox).Text
VB
CType(GridView1.Rows(YOUR_ROW_INDEX).FindControl(YOUR_CONTROL_NAME), TextBox).Text
or
CS
(GridView1.Rows[YOUR_ROW_INDEX].Cells[YOUR_CELL_INDEX].Controls(YOUR_CONTROL_INDEX) as TextBox).Text
VB
CType(GridView1.Rows[YOUR_ROW_INDEX].Cells[YOUR_CELL_INDEX].Controls(YOUR_CONTROL_INDEX), TextBox).Text
Mark it as answer if it is helpful