I downloaded the newest BTG wallet from the main webpage
https://bitcoingold.org/bitcoin-gold-wallet-v0-15-0-2-release/
and ran it with my Core wallet.dat pasted in the AppData\Roaming BTG folder (Windows)
and it showed up quickly (as soon as the blockchain downloaded) and easily, just like when I claimed my BCash.

Now just waiting for a similar Bitcoin Diamond wallet to be released.