This is occurring because your wallet is in an older format and the migrated BCH and BTG wallet funds will need to be recovered using our command line tool.
We have developed a recovery tool where basically you can recover your BCH and BTG. You will need to download Node js and npm. You can download them from https://nodejs.org/en/. Once it is downloaded, you will need to open command prompt for Windows or terminal for Mac.
npm i -g bitgo-cli
This will install our latest version of the CLI.
Login to your bitgo account by running the following and completing the steps.
Shows all the wallets owned by the user.
bitgo wallet <index of wallet e.g. 1>
Selects the wallet that you want to recover the funds from.
bitgo util recoversafehdbtg -d <your destination btg address>
bitgo util recoversafehdbch -d <your destination bch address>
With this recovery command you should be able to recover your BTG/BCH.