Gmail enables sending of e-mails with alias, which means that gmail can be sent on behalf of the custom domain setup with registrars such as GoDaddy or Google Domains.

Below is a detailed step-by-step instruction of setting up the alias for sending e-mail as service without the need of using any additional 3rd party services.

Setting up application

The first step for enabling the alias is setting up the 2-step verification and app passwords.

Navigate to Google Accounts.

Opening Google account settings
Opening Google account settings

Select the security tab and enable 2-Step Verification option. Follow the guide to setup the settings.

Google account security tab
Google account security tab

Once finished select the App passwords option to create new application. Select Other (custom name) option from the select app drop-down box.

Creating new Google App
Creating new Google App

Click Generate button. As the result the password is generated for the app. Copy this password as it will be used in the next step.

Generated password for the application
Generated password for the application

Setting up e-mail alias

Open the e-mail page and select the Settings command from the drop-down.

Opening Gmail settings page
Opening Gmail settings page

Activate Accounts and import tab.

Locate Send mail as section and click Add another e-mail address link.

Adding another e-mail alias
Adding another e-mail alias

Specify the information about the other e-mail (i.e. the one you are creating alias for).

Setting up alias details
Setting up alias details

Configure the smtp server. Set the value of SMTP server to smtp.gmail.com. User name is your gmail user name (not the custom domain name, i.e. the currently logged in user to gmail). Password is an app password generated in the previous step (not the gmail password).

Specifying SMTP settings
Specifying SMTP settings

As an optional step set the created alias as default option so e-mails will be sent using the alias by default.

Setting the default option for send as
Setting the default option for send as

Now you can send e-mails with an alias and it will appear as e-mail with custom domain to the receiver instead of the @gmail.com e-mail.