A mistake is a dead giveaway for human; proper grammar is a crapshoot; nonsensical grammar is a giveaway for gpt (like adjectives modifying words that aren't nouns, I think b/c gpt is and dropping a reference to an identifier into the comment)
I generally find spelling to be a much better tell than grammar. Most programmers I've known had (mostly) fine grammar but horrendous spelling. I frequently end up being the proofreader.