I suspect it's from "The Home Energy Diet" - see citation in this web article: www.carashops.com/article_1...ideas.html
I spent about 10 minutes on the other cited source (climatecrisis.net) but couldn't find it there.

As for how it compares to gas.... I started with michaelbluejay.com/electric...king.html for estimates of how much energy it takes to cook something with each appliance, and got these figures:
Electric oven, 1 hr, 350 degrees: 2.0 kWh
Gas oven (w/ pilot light), 1 hr, 350 degrees: 0.112 therm = 3.25 kWh
Crockpot, 7 hrs, 200 degrees: 0.7 kWh

To figure out CO2 emissions from there, www.techmind.org/energy/calcs.html has estimates (in kg) of CO2 emissions of electric vs. gas use in the United States, which comes out to approximately:
100 kWh of electric use = 54.3 kg of CO2, or 14.8 kg of carbon
100 kWh of gas use = 19 kg of C02, or 5.2 kg of carbon

So after converting everything to pounds per kWh, here's what I found:
Electric oven, for 1 hr at 350 degrees: 2.4 lbs of CO2
Gas oven, for 1 hr at 350 degrees: 0.9 lbs of CO2
Crockpot, for 7 hrs at 200 degrees: 0.8lbs of CO2

...so gas and slow cookers are fairly comparable, but for those of us with electric ovens, the crockpot is a clear winner.