Because Necrons aren't getting enough attention currently, right? In 5th edition, running your Immortals with Tesla was generally the way to go but several rule changes have muddied the waters a bit. Let's take a look at both options now and whilst the obvious place this comparison takes place is on Immortals, it can be applied to the rest of the Necron army as well.
Tesla Weapons -
With BS4, you averaged more hits than you were actually shooting thanks to Tesla's special rule (any To Hit rolls of 6 generate two extra hits automatically). Throw in twin-linking and you'd get roughly 1.3 hits per model. That's pretty awesome and made Tesla weapons the infantry option of choice since they were Assault 1 24" range weapons - they could keep away from combat and do damage to infantry pretty well. They weren't great at hurting vehicles though due to AP - but thanks to the removal of such a ruling and the inclusion of Hull Points, Tesla weapons are more capable of inflicting lasting damage on low AV vehicles by stripping hull points. They aren't going to drop a vehicle by itself (unless it's a two HP AV10 vehicle like a Venom or Vyper) but they can help strip hull points off weakened vehicles due to this new system.
Tesla is also very good whenever snap firing is involved as every time they snap fire and hit, they get three hits. This means they are much more effective than other units at dealing with flyers or in churning out Overwatch firepower - particularly if they have a twin-linked ability in there. This doesn't make them great at dropping flyers or assault units but they are a more viable option compared to straight up firepower. Remember though, they cannot deal with AV12 flyers or Dreadnoughts so their improvements in this area mean nothing against such foes.
In the end Tesla are really what they were before but with anti-vehicle improvements at lower AV values - a medium ranged weapon which is decent at affecting light vehicles and infantry on the go.
Gauss Weapons -
In 5th edition, Tesla had the mobility and range advantage being Assault 24" weapons. They could move 6" and still fire at max range but with most Gauss weapons on Infantry being Rapid Fire, any movement stopped shooting at max range. This has changed with 6th edition meaning Gauss can actually firing on the move at range now - a huge step up compared to before if it wasn't mounted on a relentless platform (i.e. Phaeron Overlord). This doesn't make Gauss as good as Tesla at range - Tesla still generates more hits and benefits more from things like twin-linking but it means Gauss isn't infinitely outclassed at 18-30" by Tesla weapons on basic infantry models.
Otherwise Gauss still has a lot more damage potential when you get in actual double tap range - the shots are doubled, shockingly, and this means lots more damage but it puts you in range of enemy assaults, etc. where Necrons generally fold quite easily. The extra damage potential is there but it's not something you want to be running towards your enemy to use. It'll more often be used defensively when the opponent gets closer but remember - the average model can now assault outside of Rapid Fire range with RCL. Be wary.
However, Gauss still has more trick up their sleeve. Hull Points. With Hull Points now making every glance a reduction in a vehicle's durability, Gauss as actual anti-vehicle weapons is a viability. Every six they roll to penetrate automatically counts as a glancing hit so even your basic Gauss Flayer can drop the mightiest of tanks. To get lots of sixes though requires lots of shots and even a 10-man Necron unit with Gauss is only going to inflict just over two Glances at double tap range. Not enough to blow a vehicle away and you're putting yourself in danger of the opponent's retaliation. With the new Rapid-Fire rules though, this ability makes Gauss based Necrons a lot more effective at range at both anti-infantry and anti-tank. They're not going to drop a tank by themselves but they can take off that final HP a more dedicated anti-tank unit may leave on a vehicle.
Gauss has really stepped it up with the inclusion of Hull Points making them a more applicable anti-tank option than Tesla across a variety of AV values whilst the changes to Rapid Fire means this can happen at range similar to Tesla weapons and still be useful against infantry. Tesla is still the better option at range against infantry though and against low AV vehicles. With the average extra shots Tesla does more damage against both targets - it's when the AV creeps above 12 that Tesla can no longer damage it or when Gauss moves into Rapid Fire range that their output becomes better. Tesla is also the best option in regards to anything involving Snap Fire - Overwatch or shooting at flyers; yes it's even better than double-tapping Gauss.
So which one to use? There's not really a clear winner here and it more depends how you want the rest of your army to work. I'd generally take Tesla on larger foot units and Gauss on units in transports. This also makes Warriors more viable as their damage is now comparable to Immortals for only a few points less.