You can not. Unless it's not a native app.
If it is a web-app, then sure you can use the power of JS to send whatever you need to any endpoint. We use Segment for real time personalization, but even that requires quite a lot of involvement from the app teams. And no, GTM is not helpful for native apps.
If you already have custom analytics tracking implemented in your app, then there's a high chance that the devs used a wrapper around the firebase/gtm sdk to send events in a more comfortable way or add adequate logging. In that wrapper then, the devs could implement the event streaming to your GCP recommendations AI in case it has the capability to store data.
If it does not, you can still set up a neat app engine (or a differnet endpoint) and stream data to it. The app engine would allow you to tweak, sanitize and debug the data, after which you can stream them to something like Big Query, which your Recommendation AI would likely to be happy to work with.
I'm talking about app engine and big query because there's a nice repo I've used a few years ago... Here: https://github.com/MarkEdmondson1234/ga-bq-stream
This repo is attempting to use front-end GTM to stream events, but you won't be able to use it (unless you're dealing with a web app). So instead, you stream them in a very similar way from your analytics wrapper.