This is something that I have brought up before when SwiftKey Flow was first introduced.
My suggestions at this point have changed a little far having some time with flow.
I have 2 suggestions for gestures with flow enabled, one is to have a gesture zone that when active provides a box to swipe in to activate a gesture then it goes away. The second one seems a little more tangible though, since there are 4 areas that are unused through flow input. The shift/caps key, the number/settings key, the delete key, and the return/emoji key. I think that a gesture that makes you press one of those keys and swipe up to activate a function would work great.
The level of intelligence that the keyboard maintains is whats important... SwiftKey to me still amazes me... I mean i can have an entire conversation working with only the 3 predictions SwiftKey has displayed