Not quite true - gasoline engine efficiency is roughly 25% while a power plant could go to 50-sh. The efficiency of transporting the power back to the electric car wheels is probably in the high 80-s low/90s. So you are ahead. Just not by that much.
Only on very new generation plants (for coal national average is cited as 32% - it may be slightly higher now)[1]. A 2016 Toyota Prius is advertised as being at 40%[2]. I remember reading a 70% ballpark for Tesla's but it might have gotten better (I can't seem to find the article now).
[1] https://www.fas.org/sgp/crs/misc/R43343.pdf
[2] http://gas2.org/2015/05/27/2016-toyota-prius-engine-achieves...
I'm not sure how much oil needs to be burned to produce 6 kWh, but since a GGE is 33.4 kWh, I'd assume it's at least 1/6th of a gallon. Gasoline emissions need to account for the oil burned to produce that heat and all energy consumed in the gasoline's storage and distribution.