The thermal gradient simply refers to the fact that the temperature will be slightly cooler by the glass and warmer in the center.
We experimented with special materials, and were able to get more uniform temperature inside the incubator and puppies hated it. Five months to develop. 10 minutes to figure out it was not going to work right. We learned a lot about puppies in the process.
Newborn puppies map out their environment using their nose. Once they find a slightly warmer spot and a slightly cooler spot, they can settle down. They move between the warmer and cooler spots to regulate temperature.
The trick is to only give the puppies good choices.