Are they "primarily driven by the ejection of gas" or more accurately preceded by gas and steam as the magma itself is gas rich and thermo and fluid dynamics dictate the gas will be at the vanguard of the breach?
The magma, like Champaign, is indeed gas rich, and when the pressure lowers, the gas is released and wants to go away, carrying lots of material with it, be it Champaign, rock or lava, that's how you get the projections (the eruption per se)