Teams supports federated guest access for Gmail accounts using the identity provider framework of Azure B2B Collaboration. Office 365 tenants must first decide if they want Gmail accounts as guests in all or some teams before going down the federation route. Why Teams and not other Office 365 apps? It’s all to do with the endpoint used by the client to connect. If it can handle federation, all good. If not, it’s standard Azure B2B Collaboration.
Microsoft is working on tools to move email, calendar, and contacts from G Suite to Exchange Online with availability in Q2 2019. It’s hardly a surprise.
Microsoft has launched the preview of Google B2B Federation, which allows Google accounts to be used to access Azure AD apps. Quite how this will work out for apps that use guest user accounts is unknown at this point.