TextBox QntTxt = (TextBox)gvShoppingCart.Rows[0].Cells[2].Controls[0];
or this
TextBox txtqnt = (TextBox)e.Row.FindControl("txtQuantity");
txtqnt.Text = txtqnt.Text;

or this
TextBox qnt = (TextBox)gvShoppingCart.FindControl("txtQuantity");
string qntt = qnt.Text;