It's all accessible, that's how Remail (not to mention iPhone, Apple Mail, etc) gets at it.

I interacted with Greplin support, they were aware searches weren't finding things that had been auto filed by Gmail rules into labels. I have been annoyed this "for pay" service didn't handle this.

(Note that I do not expose the "All Mail" virtual folder through IMAP because that ends up doubling mail.)