Make sure the e-mail is configured as IMAP in Outlook, not POP. Given the way Gmail works, there might be other factors involved, too. It would be useful to know what shows up if you log in to Gmail webmail, go to Settings, Settings choice, Forwarding and POP/IMAP tab, then the POP download and IMAP access settings.
Brian - Windows 10 Home, 64-Bit, Version 1809, Build 17763
Explanations exist; they have existed for all time; there is always a well-known solution to every human problem — neat, plausible, and wrong.
~ H.L. Mencken, AKA The Sage of Baltimore