I can now answer my own question as I have just completed the job.
Replaced both gaskets. On one side, the alternator needs to be loosened and moved out of the way the enable the cover to come out. On the other side, the air box hose needs to be removed and the coil system loosened to allow the cover to come out.
I used some gasket sealant to hold the new rubber gaskets in place on the rocker cover when replacing.
Also need some degreaser to clean the filth off the cover when done.
Just a fiddly job - took about 3 hrs. parts cosst $51 from Holden for the pair plus about 6 bucks for the gasket sealant.