Android 9 brings support for gesture-based navigation but current phones can choose to retain the conventional navigation buttons. While OEMs making Android One devices are still free to choose whatever navigation UI they deem fit, Google says that future Pixel phones will only feature the new gesture-based UI.
https://www.notebookcheck.net/Android-Pie-gesture-navigation-will-be-a-mandatory-feature-on-the-Pixel-3.321216.0.html
Now, who defines types of gestures... I expect each brand to do differently. Of course they can as well use google's type, well, they won't stand out then... Still you will have them different in any phone, hopefully it will come with manual/creative mode to make your own or what are you used to...