Mail.app not connecting to Gmail I'm running macOS Sierra on an early 2015 MacBook Air. 

I've used the Mail.app to connect to Gmail, as well as several other account profiles for years. Two days ago, the Gmail connection failed (other accounts are loading normally).

When I clicked on accounts offline in the top corner, it explained that the login to Gmail had failed, and I should re-enter my password. 

Here's the login screen it brought me to. 



Then I entered my password (which I've checked several times for correctness) and got a grey screen of death. 



This has repeated many times over the past two days, even when left alone for several minutes to resolve the grey screen.

Other attempts to re-enter the login information, for example through Mail → Accounts have led to the same result.

What can I do to resolve this?

Edit: Tried removing and re-adding the account. Now it freezes at the last step, after clicking done (shown with done pressed)
For anyone having the same problem, it turns out that Gmail blocks out-of-date mail clients from connecting and isn't too keen on informing users when it decides to do this. The solution was to update my mail client by upgrading to macOS High Sierra.