Re: GDPR
Google needs the written consent of all those that are being listened to, so it is their duty to get that consent, before they enable Continuous Match Mode in the presence of other people - or warn them and let them leave.
Also, at least in Germany, the laws regarding recording of conversations is very restrictive. You can only record someone if you tell them in advance that you will be recording, if they refuse, you cannot record. You may also only use the recording for a single specific purpose. If you say it is for training purposes only, you can't then use the recording to prove breach of contract, for example. Likewise, if Google say they are using it for the purposes of Google Assistant, they can't use it for marketing, advertising AI research etc.