As suggested by @poire-z.
https://github.com/koreader/koreader/issues/4727#issuecomment-469668338
I wanted to make the gesture a `W`, but that's a bit too unwieldy. Instead there's the suggestion of a (backward) `W`.
The default gestures for wifi on and off are diagonal multiswipe half circles, like an turnable on/off knob.