<p>Until recently we were using Inno Setup for our installations, something I would like to continue doing, unless we can get an <a href="https://stackoverflow.com/questions/61691/how-to-add-uninstall-option-in-net-setup-project">uninstall option in the start menu</a> (thanks Giovanni Galbo), however we now need to GAC some external libraries, something I suspect is only doable (or at least only supported) though the .NET Setup Project.</p>

<p>Is it possible to call a GAC'ing library from another setup application?</p>

Not sure about library, but you can call
gacutil.exe
to install/uninstall assemblies.