It depends on what you need ChatGPT to do. If you want to just have conversations with it, yes, the official app would be enough. But if you want to give it some task or goal to accomplish, it can't do that. Moreover, it's not directly integrated into the Apple ecosystem, meaning that you will still need to create a separate shortcut that uses their shortcut actions to manage your conversations. And what about the real-time web capabilities? ChatGPT iOS app doesn't have yet. Again, it depends on what you want to accomplish.