GuiStateElement element = new GuiStateElement('z', 
        new GuiStateElement.State(
                change -> {
                    change.getEvent().getWhoClicked().setFlying(true);
                    change.getEvent().getWhoClicked().sendMessage("You are now flying!");
                },
                "flyingEnabled", // a key to identify this state by
                new ItemStack(Material.WOOL, 1, 5), // the item to display as an icon
                ChatColor.GREEN + "Enable flying!", // explanation text what this element does
                "By clicking here you will start flying"
        ),
        new GuiStateElement.State(
                change -> {
                    change.getEvent().getWhoClicked().setFlying(false);
                    change.getEvent().getWhoClicked().sendMessage("You are no longer flying!");
                },
                "flyingDisabled",
                new ItemStack(Material.WOOL, 1, 14),
                ChatColor.RED + "Disable flying!",
                "By clicking here you will stop flying"
        )
);

# Set the current state
if (player.isFlying()) {
    element.setState("flyingEnabled");
} else {
    element.setState("flyingDisabled");
}

gui.addElement(element);