FWIW, various open language models are available on https://huggingface.co

GPT3 isn't available; GPT2 is, as well as BLOOM which is (supposedly) as good.

You can try them online, and you can download them. Downloading them isn't as easy as downloading as app, though: the models are pretty large and need some expertise and lots of compute to run. Though Hugging Face does offer to set you up with cloud providers.

(I haven't actually try any of this, so I don't know how easy it is in practice. I've only tried the "playgrounds" that are available with some models)

You need to give OpenAI your credit card and use their API. GPT-3 is not publicly available, and even if it were the hardware requirements for running it would be far, far beyond a typical desktop/server.

You can try it for free for three months. I wrote about how do to that here: https://simonwillison.net/2022/Jun/5/play-with-gpt3/