gianniSync allows for your progrss in gianni mangia una mela to be synced between all your devices and saved online, so that it can always be recovered.
To use gianniSync, you need to pick a server. There is an official one (more info about it).
Once you have chosen a server, you need to make an account on it and then sign in to it to get a "setup code", which contains all the information that gianni mangia una mela needs to upload your data to the server you chose.
Once you have set this up on all your devices, you're set!
Once you've made some progress, save it by selecting "Upload data" in the gianniSync menu (the two blue rotating arrows next to settings), then, when you pick up a different device, recover your progress by selecting "Download data".
If you want to create your own server, have a look at the documentation.
Please note that only the official and marmaluot uvaSync servers are managed by the creator of gianni mangia una mela. All other servers are not affiliated with the game.