The Benin international fired the Black Cats ahead in stunning fashion with 14 minutes gone on Saturday, and substitute Gardner made sure of the three points with an equally adept finish five minutes from time.
Martin O'Neill's men were forced to defend for long periods, particularly before the break, as the visitors attempted to build on last weekend's Premier League victory over Arsenal.
However, they were unable to make the pressure count when they were in the ascendancy and the home side saw out time relatively comfortably to the delight of the bulk of a crowd of 36,904.
Sunderland owner and chairman Ellis Short has written of the "tremendous impact" O'Neill has had since his arrival on Wearside in December, and the curve remains very much upward with his side having lost just two of the nine games they have played under the Ulsterman.
The Black Cats ran out determined to avenge last weekend's 1-0 reverse at Chelsea, and they started in promising fashion.
Despite the difficult conditions on a cold, blustery Wearside afternoon, they did not allow Swansea to settle into their passing game.
They might have been ahead as early as the fifth minute when Sebastian Larsson met Kieran Richardson's left-wing cross with a powerfully-struck volley at the far post, but saw his effort come back off the upright.
O'Neill was dealt a blow when he was forced to replace striker Nicklas Bendtner with teenager Connor Wickham after the Dane had been accidentally caught in the face by Angel Rangel's boot.
But Sunderland might have fallen behind within seconds of the newcomer's arrival when Gylfi Sigurdsson, not for the first-time, slipped the ball in behind full-back Richardson for Nathan Dyer to deliver a low cross.
Danny Graham failed to make contact in front of goal, but the ball ran to Scott Sinclair behind the far post and he skied his effort well over.
The visitors were made to pay within seconds when Sessegnon cut inside from the left and after exchanging passes with James McClean, curled a deft right-foot shot across keeper Michel Vorm and high into the top corner.
David Vaughan blazed over from 22 yards five minutes later as the home side threatened to kill the game off, but as the initial storm abated, the visitors belatedly found their feet.
Icelandic international Sigurdsson and Joe Allen started to get to grips with the midfield battle and as Sunderland repeatedly surrendered possession far too cheaply, it was the Welsh side who started to dominate.
Graham was unable to direct Sigurdsson's 26th-minute cross at goal, but keeper Simon Mignolet was grateful to see the midfielder's dipping effort from distance drop just wide five minutes later.
Sigurdsson saw a 39th-minute shot blocked by Lee Cattermole and scuffed a second straight at Mignolet seconds later with Sunderland pinned back deep inside their own half.
O'Neill was clearly unhappy with his side's performance as the half drew to a close, and the whistle arrived as something of a relief.
His side returned in far more positive mood to put the visitors under pressure, and they almost broke through again with 51 minutes gone.
Sessegnon's enterprising run allowed him to feed Larsson on the right and when the Swede sent in an inviting cross, the former only just failed to connect with an overhead kick.
The ball broke to Wickham, whose effort was blocked, as was Vaughan's follow-up, and Swansea eventually managed to scramble clear.
But once again, the visitors responded and after Sigurdsson had wasted a 55th-minute free-kick, he forced a good save from Mignolet with a second three minutes later.
Loan signing Josh McEachran arrived as a 66th-minute replacement for Sigurdsson with his side labouring in the search for an equaliser.
They came close to falling further behind 15 minutes from time when Sessegnon dived to meet McClean's cross with a firm header, but the ball flew just wide.
Wayne Routledge and Gardner arrived as 78th-minute replacements for Leon Britton and David Vaughan as Brendan Rodgers looked for greater creativity and O'Neill attempted to tighten things up.
Wickham came close to cementing the victory when he fired just wide from the loose ball after getting the better of an aerial challenge with Ashley Williams, but Gardner wrapped up the points with a fine volley five minutes from time.