Firstly  uncomment the line

GridView1.DataBind();  //the databind() method is required

and change the query to

SqlDataAdapter adp = new SqlDataAdapter("select * from Unit_Detail where UnitCustomerID_i = '" + i.ToString() + "'", con);

If it still does not work,change

int i=Convert.ToInt32(DropDownList1.SelectedItem.Value);