I don't see why you would classify gold as money because of this. The 5B Euros is the money part not the gold
I don't classify it as money. My central bank does that with its monetary gold.
Btw, the concept that something is as valuable as people are willing to pay for it, is semi-valid, because there's also a production cost which is real and thus rolls over to the value of the produced product/mined mineral etc. A rock may be useless intrinsically but if I traveled to the moon to get it (tremendous expenses to bring it back) the rock itself becomes quite "valuable" for some people who appreciated the effort and money spent and bought it from me with a premium.