how to use gmail as free domain email
I have quite many domains, most of them have email forwarded to my gmail box.
Most registrars have email forwarding for user's domains. By using which you can setup domain email forwarded to gmail.
Also you can use Cloudflare's free email route, which is a solid solution for simple email usage from my experience.
If you want to send domain email out, then use a specific smtp relay, which you can setup easily in gmail's interface.
For me I always use Pobox's smtp realy, which is 20 USD/y, but a pretty solid solution for sending email out.
And there are other free smtp relays, such as mailgun, sendgrid etc. You can give a try with them.