Google finally decided to roll out automatic dark/light theming to all beta users. It was only available earlier only to some users, just Google's server side things. You need Gboard's beta version 9.7 for enjoying this new feature.

Gboard will follow the system-wide theme and you need to be on a device running Android 10. There is no idea if it will make its way to Android 9 or older Android versions because they don't have automatic system-wide light/dark theme support.

 

Another thing is worth mentioning that on Android 10, if you change your theme to "System Auto", the gboard will only use default light and dark theme. You can not set which theme to use when system switches to dark mode or light mode, which is an awkward thing. Google still have chance to improve it and let users decide which theme user want in dark and light mode but I doubt Google will be listening to users this time. 

If you aren't a part of Gboard beta testing, you can grab the Gboard beta from apkmirror and install it to enjoy whatever Google is offering in this update. If you are lucky enough, you may even see the redesigned Gboard themes and Google Sans font in Gboard.

Download Gboard Beta From Apkmirror