It supports POP3 as well. But, as this thread is entitled “Mac clients for Gmail”, Postbox is really very good for that and I think worth the 30 day trial. I did use it December (I had used it long ago but had performance issues with the Mac I was using; that’s no longer an issue for my middle of the road 2016 MacBook Air with 8 GB RAM), and I think it’s very good. I still may switch to it. It handles Gmail very well, can be configure to use Gmail web keyboard shortcuts (including the “gl” to switch to another label and just “l” to set a label for a message), and is very stable.
I love the look and layout of Mail on Big Sur, but there is one thing that I miss: in Mail up through Catalina, there were collective mailboxes for all the junk, send, trash, and archive folders for all of the mail accounts you had configured. Unless I am missing something, there is no more collective junk and archive folders in Big Sur. You can create Smart Folders to try to replicate this, but they are pretty slow to refresh. Postbox also doesn’t have these collected folders, but the Smart Folders do seem to be more responsive, and there are a lot more single keystroke shortcuts compared with Mail.