Gboard may be getting an incognito mode in Android O
Marked by the familiar fedora plus glasses graphic against a dark background, incognito mode in Gboard is exactly what you'd expect – it prevents Google's keyboard from learning from your inputs, as well as from offering suggestions from it's accumulated word data base.
Using incognito mode on Gboard can't be enabled or disabled in settings as of yet, and is activated only when using the latest stable version (v. 59) or the Dev version of Google Chrome in incognito mode. The feature is currently available only in Android O DP3, so if you aren't among the eager enthusiasts enrolled in Google's Android Beta program, you'll have to wait until the final version is released. Knowing Google, the feature will likely be back-ported to older versions of Android further down the line.