In standalone/configuration/standalone-full.xml, I've declared the following host, that works
<host name="webhost" alias="gateway.example.com" />
When I try to add another alias, I'm given Error 404.
<host name="webhost" alias="gateway.example.com www.gateway.example.com"/>
The DOCUMENTATION says list of aliases is whitespace separated.
There was a bug in documentation. It is comma separated list of hosts not white space separated.
example would be
<host name="default-host" alias="localhost, some.host" >
I also fixed documentation to to address this