Which parts do I need to just replace the water pump? Gaskets, seals, o-rings? Or just the replacement GM water pump unit?

 

The new waterpump should come with 2 paper gaskets (mount between the waterpump and the block) and 2 "o-rings" (I think they're actually square in cross section) and mount on the ends of the drive gears (both waterpump drive gear and timing chain cover gear). You'll want to use teflon sealant on the six bolts that hold the waterpump to the block to ensure that the threaded holes don't leak (may have been drilled into the water jacket).

Be sure to clean all the old gasket material off the block.