Can't find documentation on how to use HeyGen with GPT as a LLM

Hi, I have created an Instant Avatar and I would like to know if it is possible to use the GPT API in order to train the avatar to my desire. I am planning on creating a medical academic expert avatar and I need to connect it to a large knowledge base, not just provide some links on the front end offered in the labs sections. Is there any documenation on how to do this?

Thank you very much :)