The world is broadening its horizons with ever more alternative energy sources. The two that I believe are most likely to over throw the "Gas Giant" are hydrogen fuel and bio-fuel. Hydrogen fuel would be relatively inexpensive to the common person and is renewable. Bio-fuels (such as those produced from switchgrass) would create a multitude of jobs and are also renewable and less expensive than gas.
Gasoline and other petroleum based fuels are a finite resource and will therefore, eventually, most likely become obsolete. This could come about in a variety of ways, from unlikely but not uncool apocalyptic oil wars, to more likely scenarios of the price crude oil becoming too high to justify using it as a fuel source.
Gas is convenient because it is easily stowed and transported (liquids are awesome for that) and is a pretty energetic fuel, but most of all because it is still relatively cheap. As the price of gas rises the use of it will decline, and it will be replaced by other substances or technology that can do the same job for cheaper.
There's a finite amount of crude oil underneath the ground, so, as the supply decreases, scarcity will increase. As crude oil becomes more scarce, the price will rise according,y until it become econmical to use alternative fuel rather than gasoline.
Peak oil doomsday scenarios are exaggerations. Gas prices should rise slowly enough so that most economies will be able to adjust.
Okay just kidding.
Seriously, we can still use it because when I googled gas, it says "Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma)". I believe we will still be breathing in some kind of gas even if oxygen becomes obsolete in 100 years from now (in which I believe breathing in solid will be more painful...)
Okay still joking :P
Here's the serious part, gas is indeed one important type of substance. In the future even if cars, airplanes and ships become useless where we have teleporters and some sort of sci-fic devices, gas can still be extracted and converted into other forms of energy.
But won't become obsolete. Why should it? Chemically speaking they are 'energy packages'. You put them in the machine, they give you energy. Such a thing can't become obsolete as long as we need energy.
A very different thing is that we will have to overcome our need of gas and find new energy resources because we will eventually run out of gas, but that doesn't make the resource 'obsolete'.