Can gases be considered as Black body?

Answer 1

Under unusual circumstances, yes. A necessary condition for an object to be a black body is that it must be "optically thick" to its own radiation. That means the photons characteristic of its black body spectrum do not penetrate far into the gas before being absorbed. For neutral gases, this would require a great mass of large diameter (like a planet) since such gas is generally highly transparent. However if the gas is ionized (aka a plasma), the free electrons scatter light more readily, so laboratory scale plasmas can be optically thick and, therefore, black bodies, if they are sufficiently dense. Z-pinch plasmas (google that) can be dense enough for that.

Answer 2

If the gas is in thermal equilibrium with the photons, yes.