GPT-6.1 Terra is available in ChatGPT Work and Codex for Plus, Pro, Enterprise, and Edu users. OpenAI doesn't list it for the Free plan in this release announcement.
You can use it through the API as gpt-6.1-terra. API pricing is $1 per million input tokens, $0.05 per million cached input tokens, and $5 per million output tokens.
For the ChatGPT Work or Codex path, you need a browser and an eligible plan. This guide doesn't include API setup; the API pricing is listed for reference.