While Gibson was certainly one of a kind, and nobody can really touch his 1968 season, unless you compare it to the rest of the league that year. (One could argue Pedro had more dominating seasons as the league was a hitter's league when he excelled.)
Gibson's best 10 years were from 1964 to 1973.
2.58 ERA (139 ERA+)
267 IP per season (average) with 32 GS'd. The WHIP of 1.099 was excellent but not the best ever.
ERA- from 1964-1973
57 Koufax
61 Wilhelm
69 Seaver
71 Gibson
10 pitchers had a better WHIP, including the players above and Marichal, Jenkins and Vida Blue. Don Drysdale was tied.
1968 was amazing. Look up his game logs and you will be shocked.
He had a 258 ERA+ and a 1.77 FIP.
However, his next highest ERA+ were 164, 151, 148 and139.
When compared to one's peers, Pedro was perhaps the best ever:
TOP ERA+ seasons:
291, 243, 219, 211, 202, 188, 163 and several more over 120.
His best 10 years (1996-2005) saw this:
2.60 ERA (177 ERA+) and a 0.992 WHIP
206 IP per season with 30 starts average.
The ERA+ margin is high enough for me to call Pedro the best of the best for any given 4 to 10 year period.