There are literally thousands of chat clients based off their API. So are they also chatGPT or the model they specifically trained and aligned for chat?
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.
Note that the API docs refer to the chat api as the "OpenAI Chat API" and not the "ChatGPT API".