When adding Gmail, one has to manually change the authentication method to "OAuth2", at least for incoming mail, though I think it should also apply for outgoing. Next, I do like how you inadvertently decided to keep the Mozilla OAuth2 keys, so Google asks me if I want to allow "Mozilla Thunderbird Mail" to access my Gmail account.

Other than that, it does get almost all the Gmail folders correct, so this is pretty much just like Thunderbird, except you had to create your own branding as you can't legally use the Thunderbird or FossaMail branding, but most of all, I don't have to endure Mozilla's tasteless* "Photon" UI! Now to try my Outlook.com account, but so far, this is better than KMail that kept giving me errors when I try to open it, oh yeah, Thunderbird too.
*in my honest opinion
Anyways, Outlook.com does work using this tutorial, though it gets a few more folders wrong, but I'm certain it's an issue inherited from Thunderbird, and nothing to do with this.