I just tried HeliBoard. Ditching Florisboard for now. I still think Florisboard has potential but it is undeveloped, and HeliBoard has everything you'd expect for a Gboard open source substitution. Let's go! Finally we have our first open source keyboard alternative without drawbacks!
Go to advanced settings on your HeliBoard app and look for "Load gesture typing library" or similar. Go to HeliBoard GitHub and look for instructions on downloading the library.
Make sure the downloaded library architecture is the specified by the message you'll see when you click "Load gesture typing library".
It's just 30 seconds and you'll be gesture typing.
I'm in the same position because FlorisBoard will over time have all the features that HeliBoard has but HeliBoard has those already so I may switch too