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If you want to access fork coins that Coinbase does not support, you need to move your BTC to a wallet of your own before the fork happens. Don't forget to account for confirmation times. A wallet of your own is a wallet like Electrum or Bitcoin Core, ie. a wallet where you have direct access to your own private keys.

After the fork you need to import these private keys (or the recovery seed words) into a wallet that supports Bitcoin Gold or B2X respectively. Using the wallet of the respective Bitcoin fork, you can freely access your coins. However you don't need to "claim" them or access them right away. As long as you have the private keys you can access your forked coins any time in the future.