Drag wants to turn your Gmail into a collaborative workspace with projects & assign.
Kiwi turns Gmail into a desktop app allowing you to use it on Windows & macOS.
Gmelius is a Gmail/Google Workspace tool that turns your inbox into CRM or support.
Sortd is an email tool that turns Gmail into an organizer for your team tasks.
Hiver wants to upgrade Gmail by allowing you to collaborate & assign emails.
Shared Inbox in Google Workspace
How to Collaborate on Emails in Google Workspace
Many tools can help you use Gmail more precisely. Tools like Gmelius, Drag, and others can convert it into a workspace where you can collaborate and assign emails.
You can create a Google group that can be used like a collaborative email account but lacks some of the additional abilities that other third-party tools provide.
Gmail can be used as a shared inbox by installing a Google Workspace app that helps you assign, collaborate and comment on emails.