How bad can it be?

When loading up the C6 today some fluid spilled out. The fluid was a darkish brown color with some chunks in it. I'm gonna get it completely rebuilt anyhow but just want as much input as I can get on this.

Chunks generally arent good. What were the chunks? Clutch material, metal, ?

That doesn't sound good at all . I would take the trans pan off and take a look before you go any further . A rebuildable core is fine but you still have to use many of the parts inside the trans . Sounds like inside problem plus maybe some rust or burned up . Brown is not a good color to come from a trans .
Just a thought !

Jim