I'd hire a carbon monoxide data logger for a week. If you've got good kitchen extraction and adequate clean air intake (good ventilation) then you should be well within any COSHH limits - assuming your gas cookers are burning properly. Once you've done a monitoring exercise then all you need to do is regular maintenance of the appliances.
Trouble with gas monitors (of any worth) is that they need regular calibration etc and are relatively expensive.

I'd start with a google search for carbon monoxide COSHH assessments or personal CO monitoring and occupational hygiene monitoring. If you have a well designed and maintained kitchen then you shouldn't really need to actually measure CO.

Not necessarily 1-wire but there are cheap easy-enough-to-use sensors available:

https://www.hobbytronics.co.uk/mq7-carbon-monoxide-sensor

No idea of the calibration available and required for this application, thought.

Our cheapish Kidde CO alarm (<£20) has a live CO level display and a peak level recording function - could you read the peak level daily or get one of the staff to read and record the peak level at the end of the daily shift? It wouldn't give you the changing levels throughout the day, but would indicate any problems with the ventilation or combustion, which might need a more thorough approach