What happened to Caesar's hostages in Gaul?

they were not killed or sold into slavery unless a breach of loyality occured. That would end Caesars ability to hold the givers in sway through them, and enrage the gauls (hostages were demanded to ensure peace) Scanning over the Commentari I have found numerous examples when gallic tribes sent mutually hostages into each others keeping when they made alliances against Rome. So apparently, giving and keeping hostages was a commonplace diplomatic practice in the region. Example:

While Caesar was in winter quarters in Hither Gaul, as we have shown above, frequent reports were brought to him, and he was also informed by letters from Labienus, that all the Belgae, who we have said are a third part of Gaul, were entering into a confederacy against the Roman people, and giving hostages to one another; that the reasons of the confederacy were these-first, because they feared that, after all [Celtic] Gaul was subdued, our army would be led against them;

(Book 2, section 1)

I have also found some examples when hostages are referenced after their taking:

He appoints Crassus over Samarobriva and assigns him a legion, because he was leaving there the baggage of the army, the hostages of the states, the public documents, and all the corn, which he had conveyed thither for passing the winter.

(book 5 section 47) So apparently the romans kept their hostages long, and left them behind under guard when speed was required.

That he confessed, that for Caesar's kindness toward him, he was very much indebted to him, inasmuch as by his aid he had been freed from a tribute which he had been accustomed to pay to the Aduatuci, his neighbors; and because his own son and the son of his brother had been sent back to him, whom, when sent in the number of hostages, the Aduatuci had detained among them in slavery and in chains.

(book 5 section 27) This shows that holding hostages in chains was not self-evident, and that sometimes they survived. (In this case their captors had to return them by the intervention of Caesar)

Another example: Caesar places the hostages in trust by a third party (a long-time client of Rome that mediated between him and the revolting Senones) (This is the case justCal mentioned in comment.)

send embassadors to Caesar for the purpose of imploring pardon; they make advances to him through the Aedui, whose state was from ancient times under the protection of Rome. Caesar readily grants them pardon, and receives their excuse, at the request of the Aedui, because he thought that the summer season was one for an impending war, not for an investigation. Having imposed one hundred hostages, he delivers these to the Aedui to be held in charge by them.

(book 6, section 4)

And the final answer: The hostages collected form all Gaul were taken by the Aedui (who betrayed their alliance with Caesar and sided with Vercingetorix) but were not released, rather used to force peoples into the revolt and to strengthen the aeduian position against Vercingetorix himself.

Noviodunum was a town of the Aedui, advantageously situated on the banks of the Loire. Caesar had conveyed hither all the hostages of Gaul, the corn, public money, a great part of his own baggage and that of his army; he had sent hither a great number of horses, which he had purchased in Italy and Spain on account of this war. When Eporedirix and Viridomarus came to this place, and received information of the disposition of the state, that Litavicus had been admitted by the Aedui into Bibracte, which is a town of the greatest importance among them, that Convictolitanis the chief magistrate and a great part of the senate had gone to meet him, that embassadors had been publicly sent to Vercingetorix to negotiate a peace and alliance; they thought that so great an opportunity ought not to be neglected. Therefore, having put to the sword the garrison of Noviodunum, and those who had assembled there for the purpose of trading or were on their march, they divided the money and horses among themselves; they took care that the hostages of the [different] states should be brought to Bibracte, to the chief magistrate;

(book 7 section 55)

The revolt of the Aedui being known, the war grows more dangerous. Embassies are sent by them in all directions: as far as they can prevail by influence, authority, or money, they strive to excite the state [to revolt]. Having got possession of the hostages whom Caesar had deposited with them, they terrify the hesitating by putting them to death. The Aedui request Vercingetorix to come to them and communicate his plans of conducting the war. On obtaining this request they insist that the chief command should be assigned to them; and when the affair became a disputed question, a council of all Gaul is summoned to Bibracte.

(book 7 section 63)