There's an article on CBSSports about how amazing Bob Gibson's 1.12 ERA in 1968 was.

Don't get me wrong, it was a great season. One of the best in the modern era, maybe the best. Gibson had a 1.12 ERA. He pitched over 300 innings. If he had thrown 1.3 more innings, he would have averaged 9 per start (he went over 9 on multiple occasions).

But, the 1.12 ERA mostly looks so amazing because of the depressed offensive levels that year. In the NL that year, the entire league had a 2.99 ERA; the AL had a 2.98 ERA. Just for a point of reference, the league ERAs last year were 4.43 and 4.50. If you make a list of the top ERA seasons since 1920, 9 of the top 50 seasons occurred in 1968, maxing out with Bob Veale and Stan Bahnsen at 2.05.

Baseball Reference has a statistic called ERA+, which adjusts ERA for league conditions and park effects. Gibson's season only ranks 4th in the modern era. Here's the top 6:

Pedro 2000 - 291
Maddux 1994 - 271
Maddux 1995 - 262
Gibson 1968 - 258
Pedro 1999 - 243
Gooden 1985 - 228

Pedro tops the list. In 2000, he had a 1.74 ERA. The league ERA was 4.91. Adjusted for Fenway, it's 5.07.

Gibson supporters will point out that he pitched far more innings than those other pitchers. But, he pitched in an era when everyone pitched more innings. In 1968, the average NL starter pitched 6.8 innings per game. In 2000, the average AL starter pitched 5.8 innings per game. Here's how the pitchers above did compared to their league (and how many games they started):

Pedro 2000 - 29 gs, 29% better than average ip/gs
Maddux 1994 - 25 gs, 33%
Maddux 1995 - 28 gs, 26%
Gibson 1968 - 34 gs, 32%
Pedro 1999 - 30 gs*, 23%
Gooden 1985 - 35 gs, 26%

Gibson does well here, the only pitcher who beats him on ip/gs% is 1994 Maddux, and it's very close. But, his edge against the rest of the pack seems a lot less significant than it does when you just look at the raw totals. Pedro also deserves to be knocked down a notch for his lack of durability. He only started 29 games in 2000, and had 29 starts and 2 relief appearances in 1999. Maddux's GS look low, but those were strike-shortened seasons. Does he deserve less credit? It's hard to say. It's not his fault that the seasons were shorter, but it's somewhat easier to deviate from the norm over shorter periods (in other words, if the season was only 1 day long, someone could have a 0.00 era).

In conclusion, Gibson clearly had one of the greatest pitching seasons of all time. There's a very good argument that it was the best. But, there's also an argument for 2000 Pedro or 1994 Maddux. Gibson had one of the best seasons ever; he was also lucky enough to have it during an incredibly favorable year for pitching statistics.