To what degree are the Iliad and Odyssey by Homer about man's relation to the gods?
The gods certainly have active roles in both the Iliad and Odyssey by Homer. Both epics start with invocations of the Muses; the Iliad begins:
SING, O GODDESS, the anger of Achilles son of Peleus, that brought countless ills upon the Achaeans. Many a brave soul did it send hurrying down to Hades, and many a hero did it yield a prey to dogs and vultures, for so were the counsels of Jove fulfilled from the day on which the son of Atreus, king of men, and great Achilles, first fell out with one another.
Although the story is about men, many of the leading characters in the epics are descended in part from the gods, and the gods, as well as inspiring the bard, routinely intervene in both stories. Odysseus' difficulties in returning home are partly due to his having offended Poseidon.
The gods also support opposing sides in the Trojan war, and cause the war to be prolonged. However, neither story is about the gods per se. Rather they are stories about humans, and the gods are normally presented mainly in terms of their effects on the humans in the story.