question

Neither Nest or Ring smoke alarm work with a GX device. The Cerbo GX has 4 digital inputs (see I/O in the settings menu). One of them can be configured as a smoke alarm. What you would need is a smoke alarm sensor with a digital output signal.

MQ-135 is a low-cost sensor for monitoring air quality and detecting dangerous gases/smoke. There are many projects around the internet interfacing with Arduino or Pi boards. however, for a simple digital output you should be able to engineer with a transistor or FET driver as a digital trigger to be monitored by the Cerbo? Of course I would not advocate using a DIY device like this as your main source of warning of a fire at home... but it could be used as an "additional" warning to a real professionally produced smoke detector (in the same location).?
The other idea would be to have a sound level detector (set to an appropriate level to avoid spurious alarms), with digital trigger output, monitoring a real smoke detector.
I know both of these options require some design/development effort but they might be the only way unless you can find a product that gives you what you need off the shelf.

If you got use Venus OS large you can probably make something with node-red and API's for smoke detectors; you might even like to make one yourself there are many DIY projects.

Just an idea, I do not have experience with this yet.

nice but that would be really a lot for me!!

@Allswell I know that in Australia I can easily buy a "relay base" for a large number of smoke alarms for sale here. The relay base simply provides a relay contact in the event of a fire alarm. This is so the smoke alarm can easily interface to third part systems in large installations. If you provide that contact closure to a digital input to the CerbobGx it will do what you want. In fact on page 35 of the CerboGX manual it very specifically mentions a fire alarm can be set as one of the four digital inputs.

Also, to be fair to Victron, it would be unusual for a mission critial system such as a fire alarm to be connected using bluetooth. It might be ok for fridge temperatures or LPG sensors but fire alarms have a different level of urgency and hence reliability is of paramount concern. It would be unusual for a fire alarm system to be interrconnected via bluetooth.

I hope this helps.