I could only enable it by switching on Google speech to text. This brings however no solution to my problem...
See attached screenshot.
It gets grayed-out when its the only keyboard selected (in other words; you actually HAVE to choose at least ONE keyboard ).
And just to get it out of the way: You have set the keyboard-app to the same language as your physical keyboard? (I assume you have, but am wondering since your Gemini is set to Dutch, and if I understand the screenshots correctly you have selected an English keyboard in the app?)
But what I THINK might be the problem, is the same as I encountered when setting up mine in Norwegian. I went through all the steps of the keyboard-configuration several times, and it refused to give the special Norwegian characters (and thus the Norwegian keyboard setting). I still had the notification about changing/setting the keyboard, and after hitting it for the umpteen time, it apparently "took" and I got my Norwegian keyboard. (I do believe I restarted my Gemini after that, but I MAY have forgotten, because after some time it restarted on its own, and I lost it again. I went through the steps several times again before it took, and then i KNOW i restarted it immediately after successfully entering the Norwegian keyboard, and it has stayed with it now even after firmware-update and several restart/crashes...) The notification disappeared after I got the keyboard set correctly.
But there is certainly some "magic" involved in getting the keyboard selection to "stick"