Actually this third-party app exists :
https://9to5google.com/2019/07/19/google-assistant-samsung-galaxy-watch/
However I tested it and I ended deinstalling it because you can't interact with everything which is related to your personal data because of Google's restrictions. I.e, you can ask when whas the Eiffel tower built but you can't ask for Google to add an event to you agenda. For me this restriction makes it useless.
Google restricts usage on devices which are not allowed (we can see this on new Huaweis, you can't install Play Store while you still are on a classic Android, even manually), so Samsung has to want to integrate Google if we want to have it on our watch, a third-party app will have restrictions no matter what. That really pisses me off and I regret not having returned my watch sooner. I used to have a Nokia (Withings) Steel HR and I had exactly the same usage as the Galaxy Watch Active I have now, but with a 20 times longer battery life.
