With Gmail shutting down Oauth2 for POP Peeper (as first announced here), here is a list of your available options to continue accessing your gmail accounts:
1) As of this writing (May 16), Gmail accounts continue to work with POP Peeper's Oauth2.
The following method is not necessary with v5.6.3
If you need to reset your Oauth2 credentials, there's a "backdoor" method to do this for POP Peeper v5.6.x:
Hold down F8 when you "Edit" the account in POP Peeper and the "Oauth2" option will be available;
e.g. select the account in the Accounts List, then hold down F8 while pressing F2 (F2 is the default "edit account" shortcut)
-> An existing password will override Oauth2, so make sure you delete any password you have entered
2) If you don't want to enable 2FA for all your accounts (which is a requirement for app-passwords, below), then you can use personal Oauth2 credentials. 2FA is now required for the account that serves your personal Oauth2 credentials. Note: Google has announced that in the near future (May 12, 2025), the account that you set this up on will require 2FA. There are some work-arounds to this requirement mentioned in the guide, so I wouldn't let this affect your decision too much.
-> This method is no longer the recommended solution as it may require that you have to re-authenticate your accounts every 7 days (this means you must perform the "reset Oauth2" procedure where POP Peeper will open your browser and you must consent to allow POP Peeper access to your accounts; this is still being investigated). As of May 16, this is not necessarily confirmed and switching to "in production" may also help avoid this.
Using personal Oauth2 credentials
3) Use an app-password. The biggest hurdle for this method is that each account using an app-password must enable 2FA.
-> If your Gmail accounts already have 2FA enabled, this method might be the easiest.
Creating app-passwords
This guide is a work-in-progress and will be updated as necessary.