Be careful doing this! Send your Bitcoins to a new wallet, before using any online tools to check your private keys for existence of BTG -- or any other forked coin.

There are malactors out there who take the private key pasted onto their "I'll check for you" site, which then drain the BTC from that private key. So, make sure there isn't any BTC in your private key, if you use such a site.

The forked coins will still exist after you've moved the BTC out, because the forked coins exist in that private key from the block they were forked on. So you could send the BTC out at any time after the fork (even, the next block! But timing that is tricky), and then retrieve the BTG later -- days, years, whenever you want.

Hope this helps!