Migrated to O365. Emails going to GMAIL Spam.

You can try checking the headers of one such email which ended up in the spam. Maybe you will find some indication (a header added by Google) why it was marked as spam. You can also try opening a support ticket with MS, but i suspect they will tell you that everything is fine on their end and will suggest to contact Google (which is tough to do). I have found such form to fill https://support.google.com/mail/contact/msgdelivery Maybe it will work.

Just guessing. But a few weeks ago we were getting a lot of phishing emails from bogus addresses, but emails were actually sent from MS servers (someone hacked outlook.com/office 365 mailboxes). If a lot of spam is sent through such Exchange Online server, then i think it can be blacklisted. Though in such case emails probably shouldn't even reach mailboxes.