Getting analog readings is something an AVR or PIC can do just as well as the Raspberry pi. However, sending these using WIFI to somewhere else is something that the RPi is much better at. Connecting an USB wireless module is easy on the 'Pi, and very difficult on the AVR or PIC family.
So, whereas you can buy say an Arduino for $15-$25, and then add stuff to get the "wireless" (another $15-$30), I'd say a RPi and a wifi module is a good (and economic) bet.
With "gas" you mean "fuel"?
For the powersupply, get a car-USB-charger that can handle 1A. Get a good quality micro USB cable. That will power you RPi.
How does the gas sensor provide its data? I'll have some cheap expansion boards available soon. (as soon as I can get things tested, and maybe I need to write a kernel driver... ).
For the WIFI you'll need an USB WIFI module. I'm not yet sure which work well. I've ordered a random $3 one from china, I'll have to see if it works. If not, too bad, $3 down the drain.
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