The original metal Necron Destroyer models didn't come with any options - they were just a Necron in a comfy chair, with a Gauss Cannon strapped to the armrest. I decided my classic Necrons needed some heavier firepower, so I sculpted myself a Heavy Gauss Cannon. After a little testing and refining, this is now available for free in the STL Downloads section.
Note that this is not a complete model, just the weapon. You will need the original metal Necron Destroyer model to carry the cannon.