You have a naked domain problem when your website cannot work without a www in front. So if you have mydomain.com as your domain, it can only be opened only by entering www.mydomain.com.
When someone enters only mydomain.com (notice the missing www in front) the error is shown.
Although there are a lot of articles on the web on how to fix this, these guides are not applicable for Google Apps anymore. Google has restricted this feature and the only way to do it was to put an A record in your DNS settings, which needs an IP address and not an alias.
Google cannot give us a specific IP address because they host their apps in different servers and it is difficult to maintain these service by hard-coded IP address.
Fortunately, I found an IP address that you can assign to your A record and it will redirect to the same domain with a www in front.
Just go to your domain hosting's admin page and look for the link where you can change your DNS Settings.
Set your "naked domain" (mydomain.com not www.mydomain.com) and point it to 188.8.131.52 and use A as its Record Type.
For some domain hosting provider, leaving the name empty or putting a @ will do as it automatically interprets it a naked domain.
After updating, have a tea break, wait for a few minutes, then test your website by typing your naked domain on your web browser.