It's already possible to swipe up from a corner to invoke the default assistant when using gesture navigation.
When Google created its Circle to Search feature, it wanted to make it as easy as possible to use. A simple long press of the gesture pill or home button invokes Circle to Search over any screen, which is really simple to do. However, by rolling out Circle to Search, Google actually took away one popular method used to invoke the default assistant -- long pressing the home button. Fortunately, Google is working on a new Android gesture that'll invoke the default assistant when using three-button navigation.