The phrase "thermal gradient" refers to a difference in temperature between two points or areas. It indicates how the temperature changes or varies across a region or substance. Full definition
Larger habitats allow you to set up a more effective thermal gradient. (petco.com)
While there is sufficient air density for this to happen (above about 0.1 bar) the gravity based thermal gradient will exist. (notrickszone.com)
Convection is not necessary for the autonomous thermal gradient which is the equilibrium state in a gravitational field as required by the Second Law of Thermodynamics. (judithcurry.com)