There is an open-source app ("Casio G-Shock Smart Sync") for syncing time with g-shock BLE watches. Some of them are also compatible with Gadgetbridge (also OS

There is an open-source app ("Casio G-Shock Smart Sync") for syncing time with g-shock BLE watches. Some of them are also compatible with Gadgetbridge (also OSS, but much richer in functionality).