These settings in Gmail will effectively allow you to send mail from your Gmail account and insure that anyone receiving email from you sees that it is coming from your domain-based email address and not your Gmail address. The following presumes the domain is protected with an SSL Certificate. Your mileage may vary. 

• First, insure that you have the domain-based email account set up and that you can access email going to that address.
• Access Gmail account
• Click the  gear in the top right.
• Select Settings

1. Select the Accounts tab.
2. Under Send mail as, click Add another email address you own.
3. Add your Name if it is not already there, and in the 'Email address' field, enter your alternate email address.
4. Uncheck Treat as an alias.
5. Click Next Step and 
   1. enter the SMTP (outgoing) server for the email you are checking for. If you are being hosted on Monterey Bay Design, your secure server SMTP setting is:
      1. mail.yourdomainname.com
      2. Port 465
   2. enter your username (enter full email address)
   3. enter password for the email address
6. Click Add Account
7. A verification email will automatically be sent to the alternate email address.
   1. Either click the link or copy and paste the verification code into the confirmation window.
8. In the Accounts and Import screen, select "Reply from the same address the message was sent to"
9. When you click Compose to write a new email, you will now be given the option of choosing the alternate email address.
   • Click the dropdown arrow next to your From email.
10. When replying, it will use the same address the message was sent to
11. Proceed to send as normal. 
12. Send a test mail to verify. 

For reference, here are the instructions in full from Google: https://support.google.com/mail/answer/22370?hl=en