Final answer:
Senator Gravel is referring to how the US government misled the American people about the true nature and success of their actions in Vietnam in the Pentagon Papers.
Step-by-step explanation:
When Senator Gravel says, “the enemy knew what we were not permitted to know” about the Pentagon Papers, he is referring to the fact that the US military and Johnson administration had misled the American people about the true nature and success of their actions in Vietnam. The Pentagon Papers, leaked by Daniel Ellsberg, revealed that the government had deliberately falsified information, such as the number of enemies killed and the impact of bombings, to paint a more positive picture of the war effort. The enemy, on the other hand, knew the truth about the situation, which was hidden from the American public.