You can read about Sign-in best practices here: Sign-in for Android games | Android game development | Android Developers
You need to do the next steps (they are async, you can’t do it in a row):
gpgs.snapshot_open() -- open or create a save
gpgs.snapshot_get_data() -- get data from the currently opened save
gpgs.snapshot_set_data() -- set data into the currently opened save
gpgs.snapshot_commit_and_close() -- commit (save) data and close the currently opened save
The full example is here: https://github.com/defold/extension-gpgs/blob/master/main/main.gui_script