Hi, your problem is different from what is discussed in this thread. You can change the unit on your G1000, from the buttons at the bottom: PFD opt → ALT Units → IN.
More on topic, a workaround, for those of you that have the 2020 version of the classic 172, is to enable it in 2024. (And disable the 2024 version) 
Now you get the gauges from the 2020 version, that shows both inHg and hPa.
As a bonus, you get instrument lights that actually work!
What’s not so great, though, is that you get the Asobo GNS 530/430, not the Working Title version.
