Do all products have to be associated with availability calendar?

There is another option and that is to make sure the product has no booking cutoff (so it can be booked today) and then use a short bit of javascript that generates today's date for you, like so...

<script type="text/javascript">

var d=new Date();
var m =((d.getMonth())+1);
var y =(d.getFullYear());
var dd = (d.getDate());

document.write("<a href=\"http://testcompany.rezgo.com/tour/6573/Awesome-Tour/6573/" + y + "-" + m + "-" + dd + "\">Gift Certificate</a>");

</script>

Make sure you replace the domain, tour id, and name with your gift certificate product. Place this bit of javascript where ever you want to link to your gift certificate. It will take the customer directly to the booking page and skip the calendar step all together.