Using Classic Outlook on Windows for personal email, calendar, and contact management
Hello
Classic Outlook and Teams are two different things when it comes to calendar.
Classic Outlook is an email client app. You can set it up to send/receive emails and sync calendar for more than one email accounts, which includes your Gmail account.
However, Teams app is not a client app. It can only support the account itself it is used to sign into. And in order for Teams calendar to work, it can only pull in calendar items from a native Microsoft domain account, such as outlook.com/hotmail.com, it doesn't support other types. It actually used to support but it got removed by Microsoft a few years ago.