Gradient is just another word for change over a distance. The ground is warm. If you could harness the energy rising from just a 2 meter patch of ground you could run a T.V. indefinitely. Since the ground stays right around freezing, warmer air temperatures and deeper, denser snow will encourage an equitemperature snowpack. This is the strengthening or rounding process.
Snow can also gain strength through the melt-freeze process.
In the spring, free water is released into the upper layers of the snow pack by the heat of the day. This snow is unstable during the afternoon when water lubricates a thick slab of individual snow crystals. After a hard night freeze, however, the snow grains are cemented together and very strong. This is why spring avalanche hazard is very predictable. If the temperature is markedly different as we pass downward through the snowpack layers, then we have a steep temperature gradient.