Anyone have an idea for how to do this with regular gmail? I have a domain that I manage via outlook.com (it's free and Google Apps isn't anymore), and I manage

Anyone have an idea for how to do this with regular gmail? I have a domain that I manage via outlook.com (it's free and Google Apps isn't anymore), and I managed to fabricate my own SPF record that authorizes both gmail and outlook senders, but I'm a bit stumped for the DKIM. Do I need to "generate" it from gmail or outlook, or both? And if so, where? Can't find the option anywhere. Maybe it's something exclusive for Google Apps?