Why not use the default and incredible, privacy protecting AOSP keyboard, and install Sayboard? It makes the little microphone on the AOSP keyboard work and gives you voice typing. It also works in incognito modes!
Its not quite the same as gboard's voice typing (no punctuation), which is a hassle at first, but when you get used to it its no problem! You just voice type in bursts, and switch back to the keyboard to add in commas and periods yourself instead of saying "period".
Try it out! I was a LONG time gboard user myself, for the voice typing and the suggestions, but with the AOSP keyboard you just add words to the dictionary to make your own suggestions. Its my favorite keyboard now. I got away from gboard because even without network permissions, its still a glorified keylogger, and to me that's unacceptable. This is kinda a out of left field suggestion to your problem, but may be exactly the thing that gets you away from one more Google app, and onto a more open source, and privacy protecting path in addition to solving your problem!