The problem with Rending Gauss would be the abnormal amount of shots you can put out with the Necron units.
With Rending, I would make all units Assault 1, keep the range, but add +1 S. Warriors would be S5AP5, Immortals S6AP4, Destroyers S7AP4 (but Assault 2), Heavy Destroyers would keep S9, but get something else - maybe instead of rending melta weaponry, or something else nifty.
And, as said before - I'd drop their armour save completely, add +1 T, and FnP. FnP serves as armour save against small arms, and against bigger stuff, you get neither. The additional point of T represents that they are made of, well, metal. Drop the points to 10-12, and give the Lord the ability to "resurrect" a certain amount of warriors per turn. Oh, and make them count-as-fearless in CC - with the current rules, it's just too simple to just run them down in CC, which is not really fitting their background...
Cheers,
-Bone