I have experimented with "Simple Keyboard Simply" and "SayBoard", both available on F-Droid. I don't love either of them.

On the Signal App, you can request the incognito keyboard mode, and my tests show that GBoard does respect that setting. But that's just for the one app.

[1] Simple Keyboard https://f-droid.org/packages/rkr.simplekeyboard.inputmethod/

[2] SayBoard https://f-droid.org/en/packages/com.elishaazaria.sayboard/