I took a good close look at the Marichal-Gibson comparison, and you might be right there, although the stats are very, very close. What's interesting is how much their careers coincided. Gibson's major inning years were 1961-1975, Marichal 1962-1975. Their 'average' years were as follows:
BG: 17-12; .591; 2.91; 17 CGs; 4 SH; 262 IP; 210 Ks, 90 BBs; ERA+ 2.89; WHIP 1.188
JM: 18-10; .631; 2.89; 18; 4; 257; 169; 52; 3.04; 1.101
So Marichal tended to win more games and lose fewer (top win seasons of 26; 25; 25; 22; 21; 21 v. 23; 22; 21; 20; 20), but had a somewhat higher field-adjusted ERA and a much lower WAR stat. I would say the WAR stat seems unexpectedly high given the other #s. Gibson had 1968 and Marichal didn't. In terms of recognition/awards they were very even. Marichal got a 83.7% HoF vote, Gibson 84%, but Marichal did hit a guy over the head with a bat, which probably cost him a few votes.
As for Koufax v. Gibson, you have to ask: peak value or career value. Koufax led the NL in ERA 5 consecutive years, BG 1x. SK had years of 27-9, 26-8, and 25-5, while BG's best years were 22-9 and 23-7. SL won three MLB CYs (only 1 given then), while BG won 1 NL CY.
Although SK never got down to a 1.12 ERA, he had three years under 2.00. In '65, when his ERA was 2.04, he had 385 Ks and 71 BBs, and averaged 10.2 Ks/9 innings. Gibson's #s were 268/62/7.9.
I remember seeing Koufax on TV as a kid, in the WSs. I would have been 10 in '66. The things I most remember were the high leg kick and the big looping curve ball. I didn't know much then but I was a big baseball fan, and I realized he was special.