If I understand correctly, you currently have a GPT created. GPTs are not accessible outside ChatGPT (on external websites).
To access GPT externally, you first have to convert it to Assistants API Assistant. You can do it manually by copying pasting instructions to https://platform.openai.com. You will then have to run the server (or integrate into existing backend) and interact with the API, create chat UI etc.
If you want to integrate easier, our startup superinterface.ai builds in this space. It gives out-of-the-box UI integration components without the need for a separate server (we host it for you). Just import your GPT or sync with Assistants API and it works.
Hope it helps and good luck!