I suspect your institution would need to subscribe to the ChatGPT API. There's a subscription fee for that, but it's not unreasonable. I think most folks using ChatGPT are using API 4.0, but ChatGPT just released 4o as well, which I think might be slightly better.
Alternatively, there are lots of LTI tools out there that are leveraging ChatGPT. By integrating those tools into Canvas, you'd get ChatGPT through those tools. The company I work for, Harmonize Learning, integrates with ChatGPT to build assignment and discussions activities, build rubrics to accompany those assignments, and uses AI to coach students against the rubrics, but there are dozens of LTI tools that do different things with ChatGPT.