We are phasing out Zendesk. Please see Discourse instead: https://community.getmailspring.com/t/mailspring-with-gmail-advanced-protection/276
Unfortunately when you enable Advanced Protection on a Gmail account, Google blocks you from connecting third party email clients that use IMAP, including Mailspring. They white-list a few very large mail programs like Apple's Mail.app and the default Windows email client, but the rest of us are out of luck!
We're hoping that Google changes this policy in the future, but the goal of Advanced Protection is to limit the exposure of your sensitive information, and forcing users to use the Gmail web interface and official Gmail mobile apps allows them the tightest possible control over access to your data.