Casio G-Shock Smart Sync
The "Casio G-Shock Smart Sync" app is the only Non-Casio app out there, which works with the actual G-Shocks and which is actively maintained, meaning it gets regular updates.

The app is developed by Ivo Zivkov and is available from Google Playstore, F-Droid or GitHub.

One big benefit of this app is, that it does not require to create a Casio ID.
Connecting a watch with the app is super straight forward. Open the app and than trigger app connection on the watch.

Casio G-Shock Smart Sync offers a lot of features, e.g.:
  • Sets watch's reminders from Google Calendar
  • Automatically sets correct timezone when travelling.
  • Trigger actions on the phone by pushing a button of the watch like taking pictures, dialling a phone number, etc.
  • Configure most watch settings from phone.
  • Sync phone alarms with watch alarms.
  • Automatic time syncronisation (only last paired watch, not multiple watches)