Some notes on GPS accuracy.
The guaranteed positional accuracy for a fixed position is less than or equal to 7.8 M at a 95% probability. Positional accuracy has no effect on time accuracy.
The guaranteed time accuracy for a fixed position is less than or equal to 40 ns relative to USNO at a 95% probability.
When a receiver outputs a NMEA sentence, it is one sentence that is a statistical best for ALL the satellites it is processing. Therefore the accuracy of a sentence depends on how many satellites the receiver can see and process. The number of satellites a receiver can see and process depends on whether the receiver is GPS only, as most older chip sets tend to be, or GNSS as well as the number of receiver channels. Older GPS only chip sets tend to only have around 12 channels while the u-blox 8 has 72 channels.
What this means in the real world is that a GPS disciplined computer clock does NOT have only one source of time, it has the total of all the satellites seen and processed.
What ultimately determines the accuracy of a GPS disciplined computer clock running ntp is how accurately it can determine that EXACTLY one second has elapsed. With USB GPS that is determined by the computer quartz crystal clock and gives you millisecond level accuracy at best. With a PPS GPS one second is determined by the PPS signal and gives you microsecond level accuracy. Given a precision oscillator, such as rubidium or cesium, you get nanosecond level accuracy.