"What gas is it?"
Translation:Vad är det för gas?
Yes, that is wrong.
"Vad ... för" is a way of asking, in this case, what kind of gas it is. It's synonymous with asking "Vilken gas är det?".
But be aware! Used differently, "vad ... för?" could be used synonymously to why.
Vad gör du det för? = Varför gör du det? = Why are you doing that?
Why "vilken gas är det?" and not "vilken gas är den?" ? isn't it clear that we are talking about an en-ord, since we are just asking which kind of gas it is?
Is this a reference to motor fuel ie US gas? In English usage we say petrol or diesel, I thought this was a question about gases such as ozone or ammonia?
You're right, in Swedish gas is only used about gases, not about motor fuel :)
We say bensin about the kind of 'gas' you use for your car, but gas refers to 'gas' as in the state of matter (gas, solid, liquid, plasma) and gases such as neon or oxygen.
So this sentence is not about car fuel. :)