That is correct. Every plugin certainly provides a list of parameters that can be controlled via host automation (i.e, accessible by the GP widget system).
The plugins that represent Audio I/O communicates with the standard audio driver provided by the OS. On OS X, it’s Core Audio. On Windows it’s typically ASIO. But those drivers do not expose audio interface specific controls (such as the RME firmware mixer stuff) and so there there is no way for Gig Performer to be aware of device specific functionality.
If there was, there would be no need for device manufacturers (such as RME) to release mechanisms such as OSC as a way to communicate with their hardware (via their own propietary software applications).
Now, some hardware synth manufacturers sometimes release a plugin (VST or AU, etc) that knows specifically about the hardware synth and those plugins will expose a set of parameters that can be controlled via host automation and then the plugin communicates with the synth hardware using whatever proprietary format they want (usually some channel or sysex messages).
It would be awesome if RME (for example) would release a plugin that knows how to communicate with RME interfaces. If they did that, then Gig Performer would be able to control that interface directly.