Yes you will need to find a wallet that supports the forked coin and passphrase in order to access the forked coins.
Also remember that with the passphrase you will be able to access any BTC as well (if present). so remember to move the BTC (if you will not use it soon I suggest paper wallet)...
Yes the first step would be to transfer BTC to a new wallet after the date of the fork. I would do that before attempting any steps to claim BTG.