It is explained in their github.
"Glide typing (only with closed source library ☹️) library not included in the app, as there is no compatible open source library available can be extracted from GApps packages ("swypelibs"), or downloaded here (click on the file and then "raw" or the tiny download button)" https://github.com/Helium314/HeliBoard#%3A%7E%3Atext=Glide+typing+%28%2Ctiny+download+button%29
In the app, you go Advanced > Load gesture typing library > Select your downloaded library.