what is the cost to replace lower intake gaskets

what are the parts going to be besides the intake gaskets

You will be draining coolant, so fresh coolant will be necessary. You will be accessing the upper radiator hose - may be a good time to replace that while you're there. Same thing with the belt and tensioner - while you're there, check those. You will need thread locker for the intake bolts on reinstallation. That's all I can think of right now...of course doing the job, something else may be noticed and advised. Also, broken bolts and a stuck EGR tube could be a problem on removal too.

should the plenum be replaced as well I have been told that was a GM mistake on that engine.

thank you could you tell me what parts are going to be needed.