why is it necessary that ionization energy is applicable to gaseous atoms only?Explain

Ionization energy can be defined as the energy required for removing the outermost electron from a gaseous atom. A "gaseous atom" means an atom that is all by itself, not hooked up to others in a solid or a liquid. When enough energy is added to an atom, the outermost electron can use that energy to pull away from the nucleus completely (or be pulled, if you want to put it that way), leaving behind a positively charged ion. Because by adding an electron to a species we make it an ion and we can’t have solid or liquid ions, so the ionization energy is only for gaseous atoms.

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