how to access an attribute of a select with jquery?

To obtain this attribute, you will have to access the selected option, but not the select itself, although this is necessary to perform the previous action. In addition, in its example the this does not refer to the select element. if you want to make reference you could pass this from the html and receive it in the function

function elegirsaldo(el){ // recibimos por parametro el elemento select
   // obtenemos la opción seleccionada .
  var saldo = $('option:selected', el).attr('parametro2');
  console.log(saldo);
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<select id="idgiftpro" onchange="elegirsaldo(this);">
  <option value="0" parametro2="0">Seleccionar</option>
  <option value="1"  parametro2="10">50</option>
  <option value="2"  parametro2="20">100</option>
  <option value="3"  parametro2="30">200</option>
</select>

Another form without reference from the html to the event.

$('#idgiftpro').change(function(e){
  var saldo = $('option:selected',this).attr('parametro2');
  console.log(saldo);
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<select id="idgiftpro" >
  <option value="0" parametro2="0">Seleccionar</option>
  <option value="1"  parametro2="10">50</option>
  <option value="2"  parametro2="20">100</option>
  <option value="3"  parametro2="30">200</option>
</select>

To load info to an HTML element, it is best to assign it to its data property.

<select id="idgifftpo" onchange="elegirsaldo(this)">
    <option value="0" data-parametro2="0">--Selecionar--</option>
    <option value="1" data-parametro2="50.00">Uno</option>
    <option value="2" data-parametro2="100.00">Dos</option>
</select>

Using jQuery you can directly access data by means of its data name , in this case parametro2.

function elegirsaldo(sel){
        var saldo = $('option:selected', sel).data("parametro2");
        alert(saldo);
     }  

Or as an attribute as you are looking for it, it must work.

function elegirsaldo(sel){
            var saldo = $('option:selected', sel).attr("data-parametro2");
            alert(saldo);
         }