"Due to org policy, you can't use (Gmail) with Teams on this device. Contact your admin"

aotaibi001 As Adam said this could be due to a policy set by an admin, this is a direct link to the info https://docs.microsoft.com/en-us/microsoftteams/sign-in-teams#how-to-restrict-sign-in-on-desktop-devices 

You can always use the web client and if necessary you don't even have to sign in to participate in a teams meeting

https://support.microsoft.com/en-us/office/join-a-meeting-without-a-teams-account-c6efc38f-4e03-4e79-b28f-e65a4c039508 

For the record I heard about this issue from another member yesterday as well and he was sure he hadn't set any restriction as he was the admin.