i've been thinking of installing a dash from a '80s grand prix in my '83 el camino. i am just wondering what items i would need from the donor car. would i need the wiring harness,hvac controls,etc.

Everything should be the same so you can get what you need.

I am using the dash assembly from an 83 Grand Prix in my 78.
The plan is to install full load in the restoration such as cruise, air, PW, PDL etc so When I removed the dash I removed it as a complete unit...disconnecting the wiring at the Fuse panel(just separating the block), steering column, A/C unit and at the harnesses going to the doors.
Then after removing the steering column(tilt wheel) some A/C ducts, speedo cable and undoing the mounting bolts, out it came as a unit.
I then cleaned it up, armouralled it and wrapped it in large plastic garbage bags until i'm ready to use it. Just make sure you label all the connectors with sturdy tags. When I go to install it , it should just be a matter of plugging the connectors back in. when I stripped the parts car I kept the door panels, carpet, windshield post mouldings, headliner and 60/40 seats...everything was like new , I will only have to respray the rear corner and rear center pieces to finish the interior. :)

my elky doesnt have power windows and it currently has dummy lights in the dash. if the donor car also has dummy lights the connections in my harness would plug right in to the gp stuff?

All though I don't know of anyone that has. I know it should.

The wiring is Toast on my Elky so I am replacing the entire front harness as well as the entire dash harness. Since I only need to unplug it at the major points (fuse panel and steering column) the swap will be fairly straight forward.
If you are doing a partial swap of say just the cluster or a partial harness you will probably want a wiring diagram for your car plus one for the doner car so you can compare color codes and accessory connections.
Sometimes they made small changes year to year, added a switch or relay somewhere and it will really mess you up.
If you are doing a total swap to the Grand Prix dash you may want to keep all the wiring intact with the dash and just unplug it at the fuse block.
Its definately not something I would want to do over a weekend.
I should be able to take some digital photos of the dash assembly later in the week and post them if that would help.