Born to the Roman Catholic branch of the Báthory family, he was closely related to four rulers of the Principality of Transylvania (a vassal state of the Ottoman Empire which had developed in the eastern territories of the medieval Kingdom of Hungary).

[11][12] Sigismund Báthory, who was a devout Catholic,[13] wanted to join the Holy League of Pope Clement VIII against the Ottoman Empire, but most Transylvanian noblemen opposed his plan.

[5] Stephen and Balthasar's brother, Cardinal Andrew Báthory (who lived in Poland), persuaded Pope Clement VIII to intervene on their behalf.

[20] Michael the Brave, Prince of Wallachia, who had joined the Holy League, invaded Transylvania and defeated Andrew with the assistance of Székely troops.

[22] Gabriel's father fled to Kővár (now Remetea Chioarului in Romania) and swore fealty to the Holy Roman Emperor, Rudolph (who was also king of Hungary), before his death on 21 February 1601.

[2] Rudolf's troops occupied Transylvania in 1603 and his officials started to confiscate the estates of noblemen through legal proceedings on false charges of treason.

[27] To avoid imprisonment, he rose up in open rebellion with the backing of the Hajdú, irregular troops stationed along the borders of Transylvania and Royal Hungary.

[30][29] Sixteen-year-old Gabriel participated in a battle against the royal army near Sárospatak in early February 1605; three years later, poet János Rimay accused him of fleeing the battlefield.

[31] Rimay also said that Gabriel spent his days mainly drinking wine and allegedly had an affair with his aunt, Kata Iffjú (who was over thirty years old at the time).

[38] Gabriel's relatives, Mihály Káthay (Bocskai's chancellor) and János Imreffy (Kata Iffjú's husband), were his principal supporters.

[40] Káthay's opponents, Simon Péchi and János Rimay, persuaded the dying (and often unconscious) Bocskai to name Bálint Drugeth his successor in his last will.

[50] Gabriel approached Rudolph I's councillors after the Diet expelled the Jesuits from Transylvania, offering to defend the Catholic Church in the principality if he ascended the throne and saying that he was ready to reconvert to Catholicism.

[64] Gabriel was ceremoniously installed in Kolozsvár on 31 March, and the Diet granted him the domains of Fogaras (now Făgăraș in Romania) and Kővár as hereditary estates.

[65] He began settling the Hajdús in Partium, and granted Böszörmény to those forced to leave Nagykálló; others received parcels in Bihar County.

[68] On 18 July, thirteen-year-old Prince of Moldavia Constantin I Movilă also acknowledged Gabriel's suzerainty and promised to pay a yearly tribute of 8,000 florins.

[75] While Gabriel was sleeping at István Kendi's home in Szék (now Sic in Romania) during the night of 10–11 March 1610, a man entered his bedroom.

[87] However, Matthias II did not ratify the agreement because it stated that Transylvania was not required to provide military assistance to Royal Hungary against the Ottomans.

[91] According to the contemporaneous Diego de Estrada, Gabriel wanted to transfer his capital to Szeben from Gyulafehérvár (now Alba Iulia in Romania), which had been destroyed during the Long Turkish War.

[91] The Diet declared Szeben capital of the principality on 17 December, limiting its privileges, authorizing noblemen to acquire real estate and Calvinist priests to preach in the town's Lutheran churches.

[95] The Ottoman governors of Buda and Temesvár (now Timișoara in Romania) invaded the Hajdú villages in Partium, forcing them to hurry back and defend their homes.

[106][107] Matthias II considered Gabriel's attack against Wallachia as treachery, because he regarded Transylvania and the two Romanian principalities as realms of the Hungarian Crown.

[114] After learning of the arrival of Ottoman troops to support Gabriel, Radu Şerban withdrew from Szeben; this forced Forgách to lift the siege.

[116] Delegates of the counties and towns of Upper Hungary persuaded Thurzó to begin negotiations with Gabriel, and their envoys signed an agreement in Tokaj in December.

[120][121] Géczi made an agreement with Michael Weiss in Brassó, however, and asked for Gabriel's removal on behalf of the Three Nations of Transylvania in Istanbul in November.

[121] After the burghers of Brassó refused to surrender, Gabriel invaded Burzenland and captured seven Saxon fortresses in late March and early April 1612.

[130] The Diet authorized Gabriel to begin negotiations with Matthias II, and their envoys signed an alliance in Pressburg (now Bratislava in Slovakia) on 24 December 1612.

[136] The Diet dethroned Gabriel on 21 October, urging him in a letter of farewell to accept the decision,[139] and elected Bethlen prince two days later.

[136] According to the contemporaneous historian Máté Szepsi Lackó, András Géczi and Miklós Abaffy soon hatched a plot to murder Gabriel in Várad.

[74] Gabriel, according to Péter, was unable to keep a balance among the different groups of the Transylvanian nobility, which gave rise to new tensions between Calvinists and Catholics, natives and newcomers, pro-Habsburg and pro-Ottoman politicians.

[144] Neither could he realize that the Ottoman Empire had recovered after the Long Turkish War by the time he ascended the throne and his expansionist ambitions were not tolerated by the Sultan.