Out-of-the-box, The Key (GCX) is the only one available.
But custom authentication can be developed, if desired. You'd need a software developer. Basically, the software developer would need to create a SOAP web service that clones the WSDL used by the built-in GCXAuthenticationService.asmx. In .NET, you could easily create a C# web sevice that does exactly this via the .NET SDK command line:
wsdl /out:C:\dev\MyCustomAuth.cs https://dataserver.tntware.com/dataserver/test/gcxauthentication/gcxauthenticationservice.asmx?WSDL
Another aproach, if you're a Delphi developer, would be to use the source code for the built-in GCX authentication:
https://dataserver.tntware.com/dataserver/test/gcxauthentication/gcxauthentication_source.zip
The built-in GCXAuthenticationService.asmx describes each method in some detail. The protocol inspired heavily by the CAS protocol, but it could be implemented against most any authentication service.
There's also an included "TestAuthenticationService.asmx" that response automatically as if the user logged in to the GCX service as test@test.com. This is how I make the demo Staff Portal available without authentication.
To configure the Staff Portal to use a custom authentication, you'd have to edit the web.config in the StaffPortal directory and in the DataQuery directory.