The six-speed manual 911 GT3 can't be sold in California... Porsche calculated the noise level to a legal limit of a standard CHP did not adopt.

at the bottom of Porsche's press release is this surprise: "The seven-speed PDK gearbox will be the only transmission offered in California with the 911 GT3."

The six-speed manual  911 GT3 can't be sold in California, sorta. It's more like they won't. 

According to a Porsche spokesperson, the GT3 passes California's drive-by noise test when equipped with the automatic; with the six-speed stick, it fails, sort of. 

 It's based on which testing method California chooses to employ.

The California Code of Regulations specifies that highway vehicles must pass a drive-by noise test designed by the Society of Automotive Engineers—specifically, SAE J1470, "Measurement of Noise Emitted by Accelerating Highway Vehicles." 

A vehicle accelerates past a microphone, and the sound pressure level, in decibels, is recorded.

SAE J1470 aims to measure "the highest noise level consistent with urban driving." The exact testing method varies based on vehicle size, power output, peak acceleration rate, and gearing, but generally, it involves a full-throttle run starting at 50 km/h (31 mph) and continuing until the engine reaches its peak-power rpm. Manual-transmission vehicles are tested in either second or third gear; given the GT3's curb weight and power output, the procedure calls for third.

Here's the thing: Automatic-transmission vehicles aren't necessarily tested at wide-open throttle. The method specified in J1470 states that "the throttle shall, as rapidly as possible, be opened as fully as will ensure maximum acceleration without operating kickdown" (emphasis added), and held at that position until the car reaches the end of the testing area. "Kickdown," as defined by the SAE, means "a forced downshift to the lowest possible gear (first or low gear)."

You see the problem here. The GT3 automatic can do nearly 80 km/h in first. Certainly, flooring the accelerator in an automatic GT3 would trigger a multi-gear downshift all the way to first. So while the procedure calls for the manual GT3 to run full-throttle nearly to redline in third, the same procedure prohibits full-throttle acceleration in the automatic version of the very same car. 

Hence, the automatic passes the test, while the manual fails.

But the SAE J1470 is no longer the most up-to-date testing method when it comes to measuring drive-by noise.

SAE J1470 was first published in late 1984. The paper reveals itself to be relatively out of date when it comes to automotive drivetrain technology: the standard test procedure assumes that the typical stick-shift vehicle is "equipped with a manually operated gearbox having not more than four forward gear ratios." 

There are alternative testing parameters for vehicles with new gearboxes with more than four forward ratios, and SAE has published updated versions of the test method since 1984. But the basic framework of J1470 seems insufficient for testing modern vehicles in 2021.

The SAE (Society of Automotive Engineers) addressed this with an entirely new vehicle standard: SAE J2805, published in 2008 and updated as recently as May 2020.

 J2805 completely revamps the drive-by test procedure. Rather than a full-throttle (or nearly full-throttle) drive-by, J2805 lays out a hugely complex method of calculating the appropriate rate of acceleration for each individual vehicle being tested. As the very first paragraph of J2805 states, this updated procedure "is based on extensive statistical investigations of vehicle operation in real traffic." J2805 includes techniques appropriate for hybrid vehicles and other advanced drivetrain designs that did not exist when J1470 method was published. Long story short, J2805 more accurately simulates "acceleration at a partial throttle condition in urban traffic."

But California doesn't use J2805. 

Vehicle noise testing is performed by CHP, as defined by California Code of Regulations Title 13, Section 1046, which specifies SAE J1470 as the official testing procedure. 

So while Porsche assumedly engineered the GT3 to pass the updated J2805 test with both available transmissions, in California, the old method is still the law of the land. (Porsche's spokesperson indicated that CHP has been planning to update its testing regulations to incorporate the new SAE procedure)

So for now, Porsche can't sell you a 911 GT3 with a six-speed manual in California—despite the fact that the automatic equipped GT3 has the same engine, the same exact exhaust system, and, makes the same amount of noise.

"We don't know when the new SAE J2805 procedure will be adopted by California, therefore we can't guarantee the sale of the manual gearbox on the 911 GT3—and where there's doubt, we're always cautious," Porsche's spokesperson told Road & Track

https://www.roadandtrack.com/news/a36741701/porsche-911-gt3-manual-fails-californias-noise-test/