A gravity gas. It would be very difused when drawn on. Ctrl draw to make it make a large square and there wouldn't only be like one gas for every 50 empty pixels. However they'd be sufficiently massive to radiate a small gravity field when Newtonian gravity is enabled. Over time they would start to fall towards each other and the more that gathered the heavier the central object and eventually all the gas would fall together making a dense solidlike material composed entirely of the collapsed gas cloud. Good to simulate the formation of stars and planets from the collapsing solar nebula. Also coliding atoms of this gravity gas would get hot. by like 100 deg per collision and eventually be THOUSANDS of degrees.
When reaching about 5000 deg they will emit PHOT and will heat themselves rapidly, only stopping when they reach maximum simulatable temperature. At this point they'll slowly disapear over a period of several minutes (assuming a frame rate of 20FPS). The description in the paragraph is designed to show how they could be used to simulate nuclear fusion.
Just kidding, that was an imitation of some dumb guy.
It would have to be an explosive gas that basically is indestructible and destroys everything in an explosion but also will get destroyed by dmnd and disappear.