There is an Fdroid app "Casio G-Shock Smart Sync" Not sure if it supports now but most likely may support in future.

This is an unofficial app for Casio G-Shock, Edifice, and Pro Trek watches. It allows you to control your watch, sync data, and configure settings without a Casio ID. It offers a faster, privacy-focused, and more powerful alternative to the official Casio app.

Key Features check ------------------ - No Account Required: Skip the Casio ID login entirely.

From https://fdroid.gitlab.io/jekyll-fdroid/packages/org.avmedia....

While I empathize with your feelings about lock-in, I encourage you to see that we live in an amazingly empowering era. I would wager that with $20 in credits for the LLM flavor of the week and and an equally expensive BLE-to-USB dongle (try the nRF52840 dongle), you will have the protocol reverse-engineered overnight. BLE protocols are often very low-hanging fruit for this type of effort.

Yes, it's annoying this stuff isn't open source from the get-go, but it has never been easier to open hardware up for yourself and others.

Isn't that common across all watches that connect to your phone these days? Please correct me if I'm wrong.

Edit: Also, not saying it's a good thing.

While Casio app may be despicable, in Android world have a look at https://gadgetbridge.org/.

Gadgetbridge is open source provides a lot of "drivers" for different BT devices from Casio, Sony, Pebble, Garmin to Xiaomi and allows to live without monstrous in size and tracking proprietary apps.

I complained to the relevant GDPR authority (German) and never heard back. It's blatantly illegal - overprocessing of PII - to require registration for connecting two devices owned by the same person, in the same room. There's no fathomable necessary or legitimate use.

ffs all it needed to do was talk to Apple Health and I’d have bought it.

If the watch gets any popular, it will take at most weeks for someone to vibecode open source app for this.