I think this is incorrect. For a scatter wound pickup like this, winding direction makes no difference. Visualize winding two pickups in opposite direction and set them side by side. They will be the same, except it changes which wire is buried at the bottom and which is on top. But that has no impact on the response of the coil to a moving magnetic field.
I think you can flip the bar magnet and swap the leads on one of the pickups and have a proper J-bass type configuration.
I think I can help clarify this question:
There are two different components to any pickup that are both sources of signal, the magnets and the coils. The pole-peices are just functionally extentions of the magnet and serve to help propagate the magnetic field through the coil.
Now the magnet is sensing the vibrations in the string, and producing a current from that, which in turn is passed through the coil through the electronics and to the amp.
The coils themselves are also a sound source though, and can act like radio antennas, picking up inducted EM waves, like radio interference (less common now that most stations have switched over to narrowband), or the 60Hz hum of the EM field produced by the wiring of the venue, stage lighting, your house, whatever.
The reason why you want the windings to be opposite (which is, in fact achieved by reversing the hot and ground leads... physically flipping the pickup over would leave the chirality of the winding the same, like the coil on a bolt remains threaded the same rotational direction no matter the orientation).
With the 60Hz hum, we aren't concerned with the magnets yet. Just the coils and the inducted radio interference that generates the 60Hz hum. Reversing the windings will put the coils out of phase with one another, so they will both produce the same radio signal but with opposite direction, effectively cancelling each other out.
Now we are concerned with the magnets and how they sense the strings.
With the magnet polarity the same, and the windings reversed, that the signal from the string will ALSO be out of phase. With both pickups on, you will get low to no signal.
How to fix that?
Reverse the polarity of the magnet. So then you get destructive interference from the radio induction, and *constructive*interference from the magnet sensing the string.
Because physics and math, it's more likely for lower frequency waves to be close to matching up than high frequency waves, and because it's nearly impossible to get exactly the same length of coilwire, you do actually get a little bit of destructive interference in the very highest register, and more constructive interference in the lowest end of the spectrum.
Which is why humbuckers tend to sound fatter.
The same thing can be acheived with a dummy coil. Gibson used to have silent P-90s by creating a cavity in the guitar where a "dummy coil" that has no magnets would live.
You could do this by buying a cheap strat pickup or P90, and knocking out all the pole peices and removing the magnets, and then depending on the polarity of the gretsch pickups, wire the strat pickup after your toggle switch (between the volume and output jack, or between tone and master vol, whatever) in a way that it is opposite of the extant pickups (so either hot or ground, depending). There is certainly plenty of space in the control cavity for this, especially where they use the cheap tiny pots on the G2220.
And that should cut down the single coil hum quite a bit no matter what pickups are selected. The orientation actually doesn't matter, and it doesn't need to be anywhere near the strings.
Back to the topic though, nobody should have to do this for a bass advertized as having humbuckers.
Even the P-Bass has a split coil which is functionally acts as a humbucker.
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