I have heard the expression "gas and alcohol don't mix" from the Egyptian president Muhamed Morsy. What does it mean?
"Gas" refers to the usual fuel for an automobile (at least the US term; in the UK it's "petrol"), and is a metaphor for driving the automobile.
"Alcohol" refers to alcoholic drinks consumed by the driver of the automobile.
This expression, then, says simply "Don't drink and drive." Or, more specifically, "Don't drive when you've had too much to drink." (The statistics for serious mishaps involving drivers in impaired condition are appalling. Please pay attention when you hear this sentence.)
This article has a good insight:
On January 30, Mursi gave a speech in Berlin with the attendance of around 200 guests invited by Körber Foundation, known for promoting social development. [...]
To prove his point of how individuals should be held accountable for their freedoms, he cited the example of “drunk driving.”
Amid the Egyptian vernacular he was using, two English words were explicitly obvious: “Isn`t there a law that a “drunk” if caught “driving” he will be arrested?” (1)
“Gas and alcohol don`t mix,” he concluded in English.
So I concur the answer above. The meaning of this phrase is "driving a car when being drunk is illegal".
(1) This tweet provides with a transcript (highlight is mine): "mesh ento betimsiko el drunk wa howa driving car wella eh?"; if there's anyone who's familiar with Arabic, feel free to update.