These gods were beings of advanced power. They could take over and possess beings, in the form of a religious fervor. They fed on belief, and when deprived of that, each other. They had no power over artificial life, like Synthoids.
In 2595, the All-High Gods took over Dellah, turning the people into religious fanatics. Anyone who disagreed was killed or forced off the planet. One God possessed Emile Mars-Smith, but he was taken off planet and starved it by trapping himself on an uninhabited planet. After the Gods had taken over Dellah, they decided to go for Thanaxos by inviting an official delegation. One God possessed Prince G'jimbo and then returned to Thanaxos, causing a religious revolution when he returned. The God was later defeated by the God within Emile. (PROSE: The Mary-Sue Extrusion)
Christine Summerfield theorised that the universe of Dellah was a bottle universe, and the Gods were Great Houses from the next universe up, trying to escape a war. (PROSE: Dead Romance) I.M. Foreman thought that the Time Lords of her universe were evacuating into her bottle universe, and she noted that, outside of their home continuum, their powers were almost godlike. (PROSE: Interference - Book One)