Giving a company my email is not implicit permission to email me whenever they want. If you actually checked the box "yes, please email me," then yes, you gave permission. But if I did no such thing, then it is spam.
> the recipient has not verifiably granted deliberate, explicit, and still-revocable permission for it to be sent.
I'm pretty sure that giving your e-mail is explicit permission and an unsubscribe link makes it still-revocable. That said I completely appreciate that emotionally it feel like spam, and I can't imagine why any business in their right mind would feel this was a good business practice.
Subscribing to messages should be explicitly opt-in, and in a bunch of countries is legally required to be opt-in. If I miss an opt-out checkbox, and you send a marketing email - that is spam by definition, if gmail decides to filter you out, then by all means it was the right thing to do; and if it was a local business, they would be fined for that.
I use (and suspect the GP does too) use something like the following definition:Spam is email you don't want - it doesn't matter if you signed up with a company or not if you don't want a particular email it is spam.
Now as a practical matter I tend to click unsubscribe if it exists, mostly because otherwise the spam filters quality drops too far, but I still consider it spam.