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 github.com/erkserkserks/openboard/tree/…/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.