If I understand the weight to be 0.5 grams, than the gold must be at least 19K or better to be worth $20US. Gold coins usually say .999 fine, which would correspond to 24K. A half gram of 24K gold is worth about $26.34US.

Bar/scrap/bullion could be anything. Just because something says $20 on it, dosn't mean it's worth $20. I suspect this person is trading on people's ignorance. Even it it was real gold, your local grocery store probably would not accept it in payment.

Compare with a krugerrand:

https://online.kitco.com/sellprice/coins/coin_krugerrand.html

Grandpa