Do you experience unfortunately settings has stopped on android device? Several factors can cause this issue such as insufficient ram due to a lot of opened apps running in the background ...

To keep Gboard from stopping unexpectedly, make sure that it's set as the default keyboard, is fully updated, and has a clear cache.

Google has kept the System UI Tuner hidden in Android Marshmallow and Android N, at least for now, simply because these settings are experimental and could have some bugs.