Turkey has officially begun membership talks with the European Union - the culmination of a 40-year campaign.
Mr Gul (L) said Turkey had embarked on a new era
Turkish Foreign Minister Abdullah Gul arrived in Luxembourg during the night for the opening ceremony, after Ankara agreed to the EU's terms for the talks.
The move followed more than 24 hours of fraught discussions among EU nations, which ended with a last-minute deal.
They ended when Austria withdrew a demand that Turkey should be offered an option short of full membership.
Turkey had flatly rejected this possibility.
Croatia also began membership talks after the UN war crimes chief prosecutor said it was cooperating "fully" with The Hague tribunal.
Despite Britain's insistence that there had been no trade-off, observers suggest Austria's concession on Turkey was linked to the decision to begin talks with its neighbour, Croatia.
The BBC's Oana Lungescu in Luxembourg says giving the go-ahead to Croatia removed Austria's last objections to negotiations with Turkey.
However, UN war crimes prosecutor Carla del Ponte rejected suggestions she had come under pressure in compiling her report on Croatia's cooperation.
A smiling Mr Gul was greeted with hugs by UK Foreign Secretary Jack Straw and other officials at a conference centre in Luxembourg.
Mr Straw, who opened the talks, urged Turkey to press ahead with reforms of the military and judiciary, and improve the situation in Kurdish areas of the country.
Mr Gul told reporters on his departure from Ankara that an "historic point has been reached today", adding that Turkey "has embarked on a new era".
"The text sets out very clearly the prospect of full membership. There is no alternative option (mentioned)," he went on.
Mr Straw, who led what he called "a pretty gruelling 30 hours of negotiations", called it a "truly historic day for Europe and the whole of the international community".
He warned it would be a "long road ahead", with negotiations expected to take about 10 years, but added, "I have no doubt that if bringing Turkey in is the prize, it is worth fighting."
'Listening to people'
Before the late breakthrough, Turkish Prime Minister Recep Tayyip Erdogan had insisted he would not compromise on his stance, "which is appropriate to Turkey's national interests and political principles".
Public still coming to terms with last EU enlargement
Tabloid newspaper campaign against Turkish membership
Governing People's Party feeling vulnerable and isolated
Element of xenophobia and Islamophobia
Memory of Ottoman sieges of Vienna
Monday saw Austria put under intense pressure, as it, alone among the EU's 25 members, demanded that the draft framework for entry talks should be rewritten.
The EU's member states must unanimously approve a negotiating mandate before talks can begin.
Austrian Foreign Minister Ursula Plassnik said her country was "listening to the people" by questioning full membership for Turkey.
"There are moments when we have to say that such fundamental things are at stake that a compromise is not possible," she warned.
But after a series of meetings with Mr Straw, it appeared she gave way.
There is deep popular opposition in Austria and other European countries to Turkey's accession to the EU, with sceptics citing Turkey's size, poverty, and main religion - Islam - as reasons to keep it at a distance.
Austrian Chancellor Wolfgang Schuessel has said he wants the EU to acknowledge popular concerns over its expansion.
But Mr Straw warned of a "theological-political divide, which could open up even further down the boundary between so-called Christian-heritage states and those of Islamic heritage".