Liquid electrolytes ionize and hence a current can pass through them. So if a gas can ionize, can it conduct electricity too? If so, what are a few such gases?
At atmospheric pressure, air and other gases are poor conductors (Insulators) of electricity. Because, they don't have any free electrons to carry current. But, once the free electrons are produced in gas by ionization (they become plasmas), electric discharge occurs.
This could be done in many ways such as by applying a large potential difference across a gas column at very low pressure, or by allowing high frequency EM-waves such as the X-Rays through the gas. This question is not proper to ask. Indeed, gases become plasmas once they're ionized!
All dielectrics have a certain value of breakdown potential. In most of them like air, ceramics, etc. (including semiconductors like Silicon), this potential provides sufficient energy to ionize a few atoms. The free electrons formed as a result of this ionization are energized enough to bump the neighborhood atoms, break some covalent bonds and produce more free electrons.
These free electrons are then accelerated by the applied electric field and they collide and ionize the other atoms to produce more free electrons (multiplication by collision). Now, there are large number of free electrons available for current flow. Thus, an electric arc is produced!
However, this isn't applicable for all gases. As @mikuszefski has mentioned, there are a few other cases such as the noble gases which are mono-atomic (and are used in most of the discharge tubes), whose breakdown potentials are only meant to ionize them!
Gases do conduct electricity, as all materials do. However, they conduct electricity so poorly that we consider them insulators. "Electricity" requires the movement of electrons. In a gas, these electrons are too dispersed to provided any measurable current. The "lightning" example is slightly different. This refers to capacitative discharge. When the two sides of a capacitor (i.e. the ground and the clouds) store too much charge, that charge eventually jumps the dielectric (i.e. the stuff in between the ground and clouds). We still don't say that the dielectric "conducts" electricity, although it obviously does. The best insulators in the world could not stop a discharge of sufficient strength. The defining quality of a conductor is that it conducts electricity "more easily" than most substances. There is no perfect conductor or perfect insulator. In short, gases can conduct electricity, but they are considered insulators for the most part.
Gas on its own cannnot conduct electricity but it can be made to conductor electricity by subjecting it under LOW PRESSURE and HIGH VOLTAGE.
Gas can conduct electricity under two condition using a discharge tube; (1)low pressure (about ~0.01mmHg) (2)high voltage (>1000v)