Claiming my BTG from my BTC private key
AFAIK there's one good wallet other than the reference client that can import the private key and make a transaction.

That's Coinomi, it can import the private key and does support BTG.

(When I wanted to dump and sell my BTG, Coinomi was great, although I needed to use the Android emulator Genymotion [because I don't have an android device])