voided cross/gammadion
I agree that the voided cross is a gammadion, composed of 4 gammas arranged in such a way that the empty space between them is cross or plus shaped.
But it is interesting to consider this:
http://www.symbols.com/encyclopedia/13/131.html
Or even a cross with a circle around it and slash through it rendering it "voided".
I don't know if Gene was ever asked about its appearance, but any of the above three would be ideal mockeries to the Christian cross and a fitting symbol for Typhon's cult with him as its Godhead.
D