Trying to add a hyphen character to a domain name in the GSP installer fails with an "incomplete" warning. After finishing the installer with a "fake" domain name and trying to change it in the management console isn't allowed either, the hyphen character is completely disabled as valid input.

So trying to use something like gsp.my-domain.tld isn't possible with GSP 3.0.3 (and maybe all previous versions, didn't test), according to RFC1034 (http://tools.ietf.org/html/rfc1034) and many registrars this should be possible. 

<quote>
The labels must follow the rules for ARPANET host names.  They must
start with a letter, end with a letter or digit, and have as interior
characters only letters, digits, and hyphen.  There are also some
restrictions on the length.  Labels must be 63 characters or less.
</quote>