System Workbench for STM32

The MQ135 appears to have both analog and digital out pins.

So this question: "Would this be an ADC project or GPIO?" is yes 😊

"how can I get a timestamp in real human readable time" - have you thought this project through?

You can use a RTC that gets "set" somehow. "Human readable" - have you heard of "sprintf" and "printf"?