GPL / Removing Attributions

and remove any attributions/license/copyright/original author info

That’s not how the GPL works. The GPL is a license that defines the terms where you may be able to modify and distribute the modified code. It’s not your work and you need to give credit the author for their work.

I’ve had completely conflicting advice on this…where can one get a definitive one? So if I wanted to run a website using one of these 100% free GPL themes, I am legally unable to change the Theme & Author Name in the style.css? And remove any license/copyright info?

Exactly.

That’s fair enough although I don’t see why any business would want to use any of these ‘free’ themes (or even buy a premium GPL one) if you’re legally not allowed to customise the files to your liking and change the Theme/Author name so it’s not obvious to everyone that you’re using a generic template…not a great look for anyone that wants to be taken seriously. It might just be easier to build html websites from scratch for any business websites and leave the WordPress templates for amateur/hobby ones.

tl;dr Don’t claim copyright on other people’s work. That’s not what the GPL is for.

You’re misunderstanding. This was part of your original ask.

I’d like to create/customise a website without people knowing which theme is being used

You can do that. Many people do.

I was curious if I can simply download any of these, freely change the Theme & Author Name in the style.css files, and remove any attributions/license/copyright/original author info

That is the part you can’t do. That’s not a GPL issue and as I wrote the GPL is a license that permits you, the user to change the code anyway you like and distribute the changed code.

That does not mean you can claim or erase credit for someone else’s work. The GPL is not a license for that and never has been. You’re asking about copyright and that’s a different topic.

I’m not talking about links to the theme author’s site. You can removed that right now, feel free. I am talking about removing credit in the style.css, PHP files, readme.txt, etc.

Taking credit for someone else’s work that way? That’s theft. Don’t do do that.

I suggest you read this. The whole works.

https://www.gnu.org/licenses/gpl-faq.en.html

I’ve had completely conflicting advice on this…where can one get a definitive one?

From a good lawyer.
And if they are telling you that you can remove copyright info, then get a better one who understands copyright and open source broadly and the GPL in particular.

You can modify the code to your liking. That is what the GPL is for. It is one of the Four Freedoms.
You cannot remove copyright information with regards to the work of others that you kept. That is not what the GPL is for.