Chat GPT is just GPT version 3.5. OpenAI released many other versions of GPT before that. In fact, Open AI became really popular around the time of the GPT 2 which was a fairly good chat model.
Also, the Transformer architecture was not created by OpenAI so LLMs were a thing way before OpenAI existed :)
Also, the Transformer architecture was not created by OpenAI so LLMs were a thing way before OpenAI existed :)