ERA+ is (including park effects):
LgERA/ERA*100 = ERA+

where LgERA is the league average ERA for that season and the ERA is the player's ERA.

According to ERA+, Bob Gibson in 1968 had an ERA that was 158% lower than the average pitcher for that year. Steve Carlton in 1972 had an ERA that was 82% lower than the average pitcher for that year. League average pitching in 1968 was 2.98 ERA. League average pitching in 1972 was 3.26.

Gibson 68:
2.98/1.12*100 = 266

Carlton 72:
3.26/1.97*100 = 165

Obviously, park effects come into play a bit here and I don't have them in front of me. For league average stats in a given year I know you can find them on FanGraphs in the menu under Leaders -> (year), and then selecting the League stats tab. I'm sure you can find them on baseball-reference, too, but I'm not sure where.

You can do a pretty cool trick with ERA+ where you estimate win percentage based off of ERA+ with this formula:

(ERA+)^2/((ERA+)^2 + 1) = W%

Bob Gibson 1968 (ERA+ 158):
1.58^2/(1.58^2 + 1) = .714
Actual W% = .710.

I have certain reservations about ERA+, but I can blab about those later.

Thanks. Blab on. It's always instructive when you do.

Gibson's 1968 season is only amplified by those numbers. I've assumed that you're not old enough to have seen him pitch that year but watch some clips. Gibson was just overwhelming between the "stuff" and the intimidating attitude.

304 2/3 IP in 34 starts... 28 complete games and 13 shutouts... 198 hits/62 BB (0.853 WHIP)... a league-leading 268 strikeouts and, of course, that 1.12 ERA. For all the loonies who think a pitcher's W-L record is meaningful, Gibson (whose team went to the World Series that year) was 22-9 in 1968.

Thirteen complete game shutouts... 22 "wins" and 9 "losses."