AFAICT, it's fully open source software (except, presumably, the nordic hal stuff?), with a JS SDK and a hosted IDE. But, I haven't used it yet because it requires WebBluetooth, and I refuse to use chrome, and I haven't figured out if the rasppi hosted IDE is still supported or not, it seems that the `nw` package doesn't support arm (anymore?).
But this was the best option I found when I was looking awhile ago.
The other things I considered were the Pinetime (software is still a bit too early from what I've seen) and "Paul's Open-Smartwatch" https://open-smartwatch.github.io/, but I really wanted GPS and that version's design isn't finished yet.