Well, plasma is not a gas technically, but Tsiolkovsky rocket equation doesn't mind if you tossing out plasma or very hot gas.
I don't know if that was at me, but nuclear pulse propulsion doesn't involve "tossing out" plasma either and although plasma is involved, throwing things out the back isn't what has the propulsive effect (which is what was implied by the post I was quoting). It involves throwing a nuke out the back and riding the explosion.
I think the point that was being made was that "riding the explosion" consists of letting "stuff" from the explosion hit the back of your thing-being-propulsed and letting it bounce off, transferring momentum to your thing-being-propulsed, which is morally equivalent to tossing things out the back of the thing-being-propulsed.
There is one subtlety here that might require a modification to the rocket equation : if the stuff being thrown out the back end is being thrown out at a relativistic velocity (e.g. is comprised of photons), then some of the assumptions in the classical derivation break down and you have to be careful with relativistic effects. This might be dealt with here
https://en.wikipedia.org/wiki/Relativistic_rocket, but I'm not sure if even that derivation deals with exhaust whose rest mass is zero (and whose velocity is the speed of light, i.e is comprised of photons).
In any event, figuring out how to have all your fuel converted into photons all of which go straight out the back is going to be the most efficient rocket you can get, and that probably CAN be analyzed using the classic Tsiolkovsky equation using an "effective" V_exhaust defined by delta_momentum_of_ship/delta_mass_used_to_get_that_delta_momentum.
[EDIT] Now that I think of it, there's another aspect of efficiency: in the photon drive case if you could arrange to consume all the fuel simultaneously then you could probably do better than the Tsiolkovsky equation, since you wouldn't be accelerating any of the fuel. This would be hard to do in practice though.
Also, "photon drive" gave me a better search term, so here's a discussion of the photon drive case:
https://en.wikipedia.org/wiki/Photon_rocket[/EDIT]
For chuckles and grins: I have a memory from my undergraduate days working in a relativity hydrodynamics group of someone saying "X has a saying that there's two kinds of stuff in the universe: gamma = 5/3 stuff (non-relativistic) and gamma = 4/3 stuff", where I don't remember who X is (but he was well known), and gamma is the adiabatic index. What he meant was that when the temperature of a gas gets so high that the thermal energy is significantly higher than the rest energy (i.e. the thermal velocities are relativistic), then the thermodynamic properties change and you have to redo the derivation, BUT the difference can be absorbed into a generalization of the adiabatic index. I suspect it's a similar effect for V_exhaust and the rocket equation. For a better discussion of the relativistic adiabatic index, see the 1st and 2nd full paragraphs on page 4 here:
https://websites.pmc.ucsc.edu/~glatz/astr_112/lectures/notes6.pdf John