There is a huge gain if English is your second language, and you use ChatGPT to rewrite, or translate.
Even if English is your mother tongue, if your written English is crappy or you need to write in a style you are unfamiliar with (e.g. formal), then ChatGPT can help.
If you're writing in a style you're unfamiliar with, how do you know the model is doing it correctly?
I also think writing yourself might be far better practice. This tool can easily become a crutch. This is unlikely to be free anytime soon. In fact it's likely to be quite expensive.
I think we all tend to be better at picking out a correct answer than generating a correct answer from scratch.
I can ask ChatGPT to rewrite an email in the style an American news reporter from 1950, and I can judge whether some of the cliches it generates feel correct. I cannot write in that style at all.
Whatever LLM search stuff comes along will only be free as long as it brings in ad revenue. Which involves making the models fundamentally worse most likely. Or they'll use it to collect personal data. Probably both.
Computationally, GPT is wildly expensive. This idea people have that it's gonna be used all over the place for all sorts of tiny tasks, as if it's just another REST API, is nuts. Unless something fundamentally new comes along that makes these models much cheaper, adoption is likely going to end up much more limited than people expect. Or siloed off into expensive business-facing products.
Even if English is your mother tongue, if your written English is crappy or you need to write in a style you are unfamiliar with (e.g. formal), then ChatGPT can help.