Can I claim my BCH if my BTC is held with Coinbase?
If you didn't move out your BTC before the fork, then your BitCoin Cash tokens remain with CoinBase. They are not really lost, but they might remain in their system for a while. Here is the explanation directly from CoinBase Support:


  We are planning to have support for bitcoin cash by January 1, 2018, assuming no additional risks emerge during that time.
  
  Once supported, customers will be able to withdraw bitcoin cash. We'll make a determination at a later date about adding trading support. In the meantime, customer bitcoin cash will remain safely stored on Coinbase.
  
  Thank you,
  
  Coinbase Team