If gesture typing is the same thing as glide/swipe (I don’t really know what the feature is called) then they have this one linked on their page https://github.com/erkserkserks/openboard/tree/master/app/src/main/jniLibs
It says they can’t bundle it with the app because they can’t find any open source ones, so the library linked isn’t FOSS I think and made by Google but it should still work if you don’t mind that.
There’s actually a video of it too that a lot of people haven’t seen (Video visuals are SFW, though there is gunfire sounds in the distance in the second half). The image alone misses a lot of what the guy did, he even climbed on top of the tank.
Youtube link: https://youtu.be/YeFzeNAHEhU?si=bdKE9yo6HukO3Fhc
Private frontend link: https://piped.syncpundit.io/watch?v=YeFzeNAHEhU