What is the difference between Open Assistant and ChatGPT?
While ChatGPT is a powerful and highly versatile tool, it is currently focused on interacting through a very simple and static text-based interface. Open Assistant, instead, aims to actively interact with third-party systems, retrieving information dynamically. 

Open Assistant is also open source, while ChatGPT is not. This allows the former to be much more versatile and elastic: it can be customized and built upon with few restrictions, for example allowing for entire ecosystems of productivity tools to flourish and more easily tackle specific needs and tasks.
Can I download ChatGPT?
No, ChatGPT can only be interacted with via the web, it is hosted remotely as part of its proprietary design.