A planet warms until it radiates away exactly as much energy as it soaks up. Sunlight in is fixed; the surface pays it back as infrared, and its emission climbs steeply with temperature — Stefan & Boltzmann's σT⁴.
Greenhouse gas doesn't heat the air like a stove. It is nearly transparent to sunlight but opaque to infrared: it intercepts outgoing heat and re-radiates half of it back down. Less escapes for a given temperature, so the surface must warm until its σT⁴ pushes enough infrared through the thicker layer to balance the books again.
That is the honest mechanism — not "hot gas," but a new equilibrium temperature where incoming and outgoing photons match. Add gas and the balance point moves up; thin it out and the planet cools straight back down.
The same accounting, run on real numbers, is why Earth sits near 15 °C instead of a frozen −18 °C, and why Venus's thick blanket bakes at 460 °C.
Something in the simulation stopped unexpectedly — the lesson continues without it. Nothing you did was wrong; you can move on.