What worked for me was the following:

Turn on 2FA for your Google account if it's not on yet.

Go to accounts.google.com -> Security -> Signing in to Google -> App Passwords.

Generate a new password, choose "Other" as the name and enter something like KMail.

In KMail, after you get this error, close all extra windows and go to Settings -> Configure KMail -> Accounts -> Receiving -> [Click on whichever one you want to set up] -> Modify... -> Advanced -> Authentication and choose PLAIN

After I clicked OK, I got a pop-up saying that Gmail refused to authenticate with the given username/password. I then went to authenticate again using a button in that dialog box. If you don't get this, find some other way to get to that point.

This time, after entering your e-mail address and verifying 2FA, DON'T use your actual password. Instead, use the App Password generated in step 3. Just enter it into the password field and press Enter.

KMail should now wor