Now it's possibile to load and save data from json using ofxJsonSettings custom fork.
// Load settings
gui->loadSettings("data.json");
or, when you have multiple gui you can load the "data.json" the first time and after use the loaded data for all the other gui:
gui1->loadSettings("data.json"); // First load the "global" data for all the gui and use for the gui1
gui2->loadSettings(); // than use the data just loaded also for gui2
// Save settings
gui->saveSettings("data.json");
or
gui->saveSettings(); // Save sata using the name of the last loaded json