To add a Gmail account to eM Client as POP3 with OAuth authentication, use the manual account setup:

1. Go to Menu > Accounts (Windows) or eM Client > Accounts (macOS)
2. Click '+Add account'.
3. In the New Account window, select Mail > Other.
4. Insert your Gmail account address.
5. For the incoming POP3 server, enter: pop.gmail.com
6. For the outgoing server, enter: smtp.gmail.com
7. Continue the setup. You will be redirected to your browser to grant access via OAuth.