I *May* have to switch too when Microsoft cuts off the old sign in this Sept. -- Mail can allegedly handle the required advanced log in, but I've been using Mail since the Win10 beta, *probably* set up the accounts using the old method, and *may* not be able to set up Gmail if I have to redo it using the advanced sign in. Running Win11 23H2, last month the new outlook took over, so I installed Mail from the Store, which worked, except I couldn't get Gmail working, so I restored a recent backup.
Then yesterday the new outlook took over, and the toggle to go back to Mail had been removed -- a one-time message window said you could still swap back in Settings [if I remember correctly]. When I couldn't find anything in Settings, I found that I could edit C:\ Users\ [UserName]\ AppData\ Local\ Packages\ microsoft.windowscommunicationsapps_8wekyb3d8bbwe\ LocalState\ Migration\ callback.txt to read: "permaToggle":false,"allowAutoToggle":false". *So far* that's gotten Mail back & working, with fingers crossed it stays that way.