Status update for anyone who finds this thread:
There’s still no official ChatGPT link, but David said on Sept 9 that an official Intervals.icu MCP server is at the spec stage: Request for Official MCP Support for AI Tools (ChatGPT, Claude, ...) - #12 by david
For ChatGPT specifically, whether you can plug in your own MCP server depends on your ChatGPT plan, and OpenAI changes it often. Their page is the current word: https://help.openai.com/en/articles/12584461-developer-mode-apps-and-full-mcp-connectors-in-chatgpt-beta
Until the official one lands, there’s Marc’s self-hosted server above, and a few hosted ones in AI Tools: AI Tools - Intervals.icu Forum
Disclosure: I built one of the hosted ones (IntervalSync), so take that last line with that in mind.