It depends where you are. Some people do strongly see it as not gender-neutral, and will loudly criticise you for using it even if you mean it in a gender-neutral way. Which is a shame, because while language is important there are probably other bits of language that are worse.

Ireland has "hey lads" as a gender-neutral term which surprised me the first few times I heard it.

> Those really smart guys (and, unfortunately it is mostly guys and not gals)

To be fair, I'm not sure why the original poster included the gender here instead of saying "smart people." The gender thing is a bit off topic.

You are right that "guys" is often accepted ...at least as an informal second person plural. I get the impression this is because English just doesn't have a lot of great options here. Personally, I use "y'all." No matter the company you keep (with the possible exception of a group of CEOs) it works and doesn't make things awkward. That said, contractions like "Y'all'd've" start to get a bit messy and I generally stay away from those ...especially when writing.

What's wrong with the second person plural subjunctive past perfect?

As an American speaker who grew up in the south and lives in the Midwest, that's not true in every case. When addressing a mixed group, "hey guys" can usually work. When talking about a group of unspecific people, it can work, but mostly because our cultural default is to assume male. When talking about a group of specific individuals "the guys in the office are smart" I would take that to be more specifically male. It's definitely not unambiguously mixed gender. In the case of the GP, the word was used to make a point about the ambiguity of the word, in a way that's very naturally idiomatic for a native English speaker, imo.

As another native speaker I can say that "guys" refers to a mixed gender group perfectly well. Saying that it implies a predominantly male group is more indicative of your personal life experiences or bias.

Regarding GP, my only guess is that they were virtue signaling because they explicitly went out of their way to make that point. If they really wanted to avoid gendered nouns they could have written "folks", "people", or any number of similar words (kind of like how I'm doing here, by not assuming GP's gender).

'Folk' is fine, though try to work out how to drop 'comrade' into your vocab.

In this particular case, the OP could have used 'researchers' and: 1. avoided putting off/distracting people by the use of gendered language; 2. been more precise.

Also a native speaker, here.

Seems to me that "guys" can refer to a group of men, or to a mixed-gender group, but not to a group of women. So while it's absolutely the case that it can refer to a mixed-gender group, using it that way is still problematic in that it emphasizes men's membership.

I use "guys" to refer to groups of women all the time. I don't think people who say "you guys" check and make sure it isn't 100% women before saying it.