Bob, I've been checking the difference in these 2 model dimensions.
1. I see the length of the radiators is 1" longer on the model Homer posted, but it has a top hat too, and the physical gamma model has no top hat.
2.The model Homer posted has the average radiator diameter set at 0.75" inches and the model with the physical gamma is set at 0.80" inches.
3. The loop diameters are identical at 29.97" inches.
4. The radials lengths are also identical at 106.259" inches.
5. The radial off-set at the hub bracket for both is set at 2.50" inches and I've had to guess at this dimension. I forgot to check the off-set for the wire to the feed point, but I think it is close to < 2" inches...but I used my Sigma 4 model for this measurement, so it is just a guess too regarding the NV4K.
6. The real gamma model uses 313 segments and Homer's model has 453 segments. That could have the effect of the difference in gain reported. 
So IMO, with these dimension being so close, it appears that the difference in reported gain has to do with the matching differences or the segments difference.
This leaves me to at least consider, that we can't just take some dimensions and make a model that looks like some antenna we saw somewhere at a distance, and expect to see really good results.
