not the GP but I'm a fan of the Netatmo Indoor Air Quality Monitor. Price is fair imo (100usd), the app works well plus the data can be accessed via the web, and it includes humidity, temperature, and noise monitoring in addition to CO2 monitoring.
i only bought TFA Dostmann AirControl Mini CO2 Meter TFA 31.5006 Ambient Air Monitoring by accident, it does what it does and is super unflashy so i got another one a year later.
The indoor kits include a high quality NDIR CO2 Sensor from senseair.
[1] https://www.airgradient.com/kits/