Custom Sounds is used to add your own sounds to a graphic set, not for extra default sounds that don't have a name.
Anyway, currently GSTool does not have any sound library included; though it shouldn't be too hard to include one. With that being said, it's likely the current GSTool will need to be either heavily modified or just outright replaced for new formats, unlike the other tools which could more easily be adapted to it, so I'm reluctant to make any major changes.