If I understand you correctly, you are asking whether you earn points through paying for a Eftpos gift card with your David Jones Amex Card.
In my experience, it depends on where you buy the card from. E.g.: If I buy a $100 Bunnings gift card from Woolworths and I pay with my Plat Edge card which gives me 3 points per $ spent at supermarkets, I get 300 points.
I must say I am not 100% sure as I have only bought prepaid Mastercards from Woolworths and I do not have a DJ card.
However, IMHO, I think you do earn points if the retailer gives you points for $ spent.
My 2 cts