From the gvinum(8) man page:
stop [-f] [volume | plex | subdisk]
Terminate access to the objects, or stop gvinum if no parameters
A quick look at the implementation however shows that the stop command never even attempts to look at any parameters and always falls back to unloading the entire kernel module.
This is quite unfortunate in practice as I have no idea how one is supposed to gracefully detach objects (removing a volume, removing a plex, etc) since the detach command refuses to operate on running objects. The setstate command helps but considering that the docs say it is meant for diagnostics only and I have already panicked my kernel once while testing it I'd rather not use it in a production environment.