I've just joined this list. I don't really go for nicknames, but careful examination of my e-mail address would reveal my Whorl name, if I had one.
As I was reading, my mental image of the gammadion was (eventually):
XXXXXXXXX
XXXXXXXXX
XXXXXXXXX
XXXXX XX XX
XXXXX XX XX
XXXXX XX XX
XXXXXXXXXXX XXXXXXXXXX
XXX XX
XXXXXXXXXXX XXXXXXXXXX
XX XX XXXX
XX XX XXXX
XX XX XXXX
XXXXXXXXX
XXXXXXXXX
XXXXXXXXX
(only square rather than rectangular).
Reasons: the religion is supposed to be an evil one, constructed as a mockery of the real religion when the Whorl left on its journey. The swastika is the biggest symbol of evil the world has seen, and was also an early Christian symbol (reinterpreted as an Aryan supremacist symbol by the Nazis, but not invented /ab nihilo/ by them).
Wolfe's narrative technique often consists of describing a scene in neutral terms, to form an image in the reader's head that creeps up on him or her at an unexpected moment. Examples: the description of the picture of Neil Armstrong on the Moon in the library in the Matachin Tower (in tSotT), the description of the tower itself as a spaceship.
So, the reader at some point realises that the "holy" men are actually venerating the swastika.
The void in the centre is there to provide a justification for "the sign of addition" replacing Christian crossing---another perversion.