Danny, K6MHE wrote:
"I wonder what his 6 dB gain is"

There is a "Super J" antenna on page 16-27 of the 19th edition of the
"ARRL Antenna Book". The book says the two stacked 1/2-wave collinear
elements provide about 6 dB gain over a 1/4-wave whip antenna and about
3 dB gain over a 5/8-wave antenna.

I won`t speculate that you actually get more gain from (2) stacked
5/8-wave sections, but it`s possible. Maybe 3 dB more, like a single
5/8-wave over a single 1/4-wave.

6 dB over a 1/4-wave whip is 8.14 dBi.
The 1/4-wave whip has a gain of 2.14 dBi

Best regards, Richard Harrison, KB5WZI