Reuben took advantage of being allowed acres of space in the Senegalese box to head in Ikechukwu Ibenegbu's tasty delivery from a free-kick, and Okpako finished off a sweeping counter-attack with three minutes of time left.
Overwhelming favourites to win the tournament after having beaten the Senegalese during the group stages, the Nigerians took some time to settle into the game, as the Teranga Lions poured forward in the opening minutes.
But the Super Eagles gained composure as the game progressed and gradually worked their way into the encounter, with Ahmed Musa a threat down the wide areas and Ibenegbu outstanding in his creative midfield role.
But despite their dominance, the Nigerians failed to find a goal in the first half, with ThankGod Ike's header cleared off the line the best chance of the half.
All that was to change barely five minutes into the second half. Musa streaked down the left side with intent, and had to be hauled down for a free-kick. Ibenegbu's delivery was inch-perfect and Gabriel must have been as surprised as anyone when he found himself with acres of real estate two yards out.
That did not stop him from heading in for Nigeria's opener.
The goal sparked the Lions into life and a five-minute spell of sustained pressure was only rebuffed by some desperate defending from the hosts.
All that was to prove academic when substitutes Chinedu Ezimorah and Ejike Uzoenyi combined to set up arriving skipper Okpako for the simplest of tap-ins and wrap up the win.