This article will explain how to set up Google apps Gmail with your 123-reg domain.
Select the Domain from the drop down box you wish to assign to Google apps Gmail.
Click on the Manage icon.
Click on the Manage DNS icon under the Advanced domain settings section .
Click on the Advanced DNS tab.
Now delete any existing records by clicking on delete buttons next to the records you wish to delete.
Scroll down and input the new MX Records supplied by Google and click the Update DNS button.
The MX records on your domain name will now be setup to use Google apps Gmail
Please Note: It may take between 24 - 48 hours to propagate the changes