Alma is perhaps a little less clear here than he has been before. He has argued that the Fall required the ability of man to die, so that his state might be changed. He is now arguing that after God provided for the Atonement, it was important that God tell man about it.
After the Fall, God sent angels to converse with Adam and Eve to teach them about the plan of redemption. After creating the conditions for the Fall, and the conditions for the redemption from the Fall, God informed man of what had been done, and what man's role would be. This is the revelation of the gospel to man.