You need to have bitcoin to the address which you are holding the private key before the block 491,407 is mined. After that, you will have bitcoin gold. Use the private key of the address which you store your bitcoin before block 491,407 and import it to the bitcoin gold wallet to receive bitcoin gold.
Butcoin gold is a forked coin of bitcoin itself.
And to claim bitcoin gold right now the most trused exchange is
yobit you have to sumbit your public key there.
but i personally donot trust yobit ,i preffer to wait for btgs own wallet to come and then i will claim my btg through their own wallet.
I hope you will wait too.Donot risk your bitcoins for gold..