Do multiple GG or GA bonuses stack?

Just wondering, like for instance china, what benefit would having a bunch of Great General or Admirals be.

No, because they affect different units.

Edit: Sorry, I just realised what you meant. I don't think so, but you want more great generals for extended range.

Individual combat units do not benefit from multiple GGs or GAs (i.e., no stacking of GG/GA combat modifiers), but as Viregel notes, there can be strong benefits from multiple GGs/GAs -- to cover more of the battle space, to accompany different armies/navies attacking different targets, and, in the case of GGs, for strategic placement of citadels while retaining other GGs for GG combat bonuses.