How are you using ChatGPT? AFAIK, there's no API for ChatGPT yet.
It uses a lurky captcha solving service, as well as chromedriver, which bin patches chrome so that chrome is automatable in a hard to detect way (off the shelf selenium is detectable by websites), but it works quite well, if you can manage to get it running. We've got a ChatGPT slackbot setup, which is quite a bit of fun in a group setting.
A quick google search reveals GPT-Neo and GPT-J as being open source. But haven't yet tried them, so can't vouch if they're as good.