Using GBoard and related keyboards, it is impossible to input Chinese, Korean, Japanese and similar characters into the KWGT "formula" field natively. These characters can still be used by copying/pasting them into this field, however when entering the field, the keyboard is forced into roman-character-only-mode and this cannot be overridden either from within KWGT or GBoard's settings.
I believe this to be an oversight, and am very surprised this has not been mentioned before.
Edit: it appears Hindi input works. (thanks to @Spiritsmaker#6602 from Kustom Discord)
Relevant system info:
KWGT Pro; APK version 3.38b903018 (latest)
Android 9.0; OxygenOS 9.0.12 (latest, stable)
Device: OnePlus 6T (A6013, stock firmware)