The Turkish government has called it a "terror attack" and suggested it was carried out by a suicide bomber.
Gaziantep is 64km (40 miles) from the Syrian border.
Deputy Prime Minister Mehmet Simsek called the attack "barbaric" but said: "God willing, we will overcome."
It happened in a part of the city where many university students live, after a wedding party spilled out on to the streets. The blast was heard across town.
group has said it was behind the bombing - but government sources say it could have been carried out by so-called Islamic State, which is known to have operatives in the border city of Gaziantep.
Turkey has been hit by a series of bombs both by IS and Kurdish militants in the past year, with the last IS attack on Istanbul airport in June, killing more than 50 people.
The jihadists have recently lost ground in northern Syria, including a former stronghold, Manbij. Syrian rebel soldiers are preparing to advance further into the IS-held province of Jarablus.
If this bomb was the work of IS, there will be speculation it is a revenge attack, intended as a show of strength by a group on the defensive.The BBC's Seref Isler, who is from Gaziantep, said the attack "took place in a city that is already on edge because of what's happening right across the border" but was even more shocking because a wedding party was targeted.
He said: "Weddings are in Turkey considered sacred and very happy occasions, so to intentionally turn it in to a bloodbath has received some very staunch criticism to say the least.
"Turkish society seems to have been horrified that this has targeted specifically a wedding, what should have been the happiest day of this couple's lives."
Southern Turkey has been hit by several deadly blasts over the past year, linked either to Kurdish separatist militants or so-called Islamic State (IS).
Often the attacks were linked to developments in the war raging in Syria.
A suicide bomber believed to have links to IS killed two policemen in Gaziantepin May.
In northern Syria, just south of Gaziantep, there has been heavy fighting between IS jihadist militants and Syrian Kurdish forces known as the YPG.
The border town of Kobane was wrested back from IS control by Kurdish-led forces in January 2015, after months of bitter fighting.
There has been a spate of bombings in eastern Turkey in recent months blamed on the rebel Kurdistan Workers' Party (PKK).
Earlier on Saturday, Turkey's government said the country would take a more active role in efforts to end the war in Syria. Prime Minister Binali Yildirim said a future political settlement for Syria must not include Mr Assad, the PKK or IS.
"In the six months ahead of us, we shall be playing a more active role," Mr Yildirim said. "It means not allowing Syria to be divided along ethnic lines."
