oh here it is. I was not convinced. If you say that external gravitational field can affect the internal energy. Then so can psuedo gravitational field due to gas in a accerlating frame. But I have read it essentially many times that systematic motion of a gas as a whole cant change the internal energy of a gas. You can see that in wikipedia as well.
You can read this " It is the energy needed to create the system, but excludes the energy to displace the system's surroundings, any energy associated with a move as a whole, or due to external force fields." Taken from http://en.wikipedia.org/wiki/Internal_energy