The French striker spurned several chances earlier in the game, but got on the end of Jefferson Montero's cross to bag Swansea an 87th-minute winner.
The result leaves Villa only three points above the relegation zone and takes the Welsh outfit onto 43 points, four short of their record total in the Premier League.
Below, Sports Mole analyses the events at Villa Park to see if the result was reflective of the action.
On target: 3
On target: 6
Was the result fair?
Swansea certainly deserved to be ahead at the break, but Villa bossed the majority of the second half and will feel aggrieved not to have secured a point. They appeared the more likely to win the game before Christian Benteke's injury withdrawal sent them back into their shells somewhat, and Gomis finally found his radar to earn his side victory.
While providing a constant threat on the counter-attack in the opening 45 minutes, Villa's attack lacked the conviction that has been integral to their recent improvement. Tim Sherwood must have encouraged them to be positive during his half-time team talk, because the hosts immediately upped the tempo and started getting their full-backs forward as they did in the thrashing of Sunderland.
They dominated possession and territory in minutes 45-70, but Gabriel Agbonlahor and Scott Sinclair squandered the half-chances that they created and Gomis hit Villa with a late sucker-punch. Today was a reminder that not all of their problems have been solved.
Created all of the first half's best chances, two of which were wasted by Gomis and the other forced Ciaran Clark into a goal-saving clearance off the line from Wayne Routledge's shot. Their defence stood up to several quick Villa breaks, but were finding themselves coming under pressure midway through the second period.
Garry Monk's clever introduction of Montero gave the Swans an outlet and an avenue to exploit the space in behind the rampaging Leandro Bacuna. The Ecuadorian's cross, following Routledge's delightful outside-of-the-foot pass, provided Gomis with his fourth bite at the cherry and the decisive bite in this match.
Sports Mole's man of the match
Jefferson Montero: Villa's Fabian Delph was a shoe-in for this award prior to Gomis's late winner, after an industrious and complete midfield performance, but Montero made a game-changing impact off the bench. He got in behind Bacuna on several occasions and his perfect first-time cross supplied Gomis with Swansea's late winner.
Gomis eventually got his goal, but it appeared for a long time as though it would not be his day. He squandered a handful of chances, the first and easiest of which was when he tamely struck Neil Taylor's cross straight at Guzan from six yards.
Sarcastic cheers greeted Robert Madley's award of a late Villa free kick, but the home side can have no real complaints about his performance. Minor decisions here and there went against them, but Madley had a solid game on the whole, certainly compared to some of his colleagues this afternoon.
Villa: Sherwood's men begin a run of crucial fixtures when they travel to Manchester United after the international break.
Swansea: The Villans will be hoping that Swansea can do them a favour when they host Hull City in a fortnight.