chatGPT definitely refers to the frontend since their API uses the actual model name (gpt-3.5-turbo) chatGPT isn’t referenced. And their API has been spotty/down all morning.
Note that the API docs refer to the chat api as the "OpenAI Chat API" and not the "ChatGPT API".
OpenAI also has an API which includes chat completion (using GPT models), image generation (DALL-E 2), and more. This has also been down this morning and is still super spotty.
Other websites using “GPT” in their name are probably using their GPT models through their API and/or are riding on chatGPT’s incredible success and public exposure.