In Also in 1913, Gabriel was [[ordination|ordained]] a priest in the Uniate Church in Lvov. After his ordination he served in the [[Cathedral]] of the Transfiguration in Lvov as well as . He also became a professor of theology and philosophy at the Greek Catholic Theological Seminary in Lvov academy. In the late 1920s Gabriel emerged as a critic of the Vatican's Uniate policy and the leading representative of the ‘Eastern’ (anti-[[Latinization]]1920–8) orientation among and the [[w:Ukrainian Catholic University|Greek CatholicTheological Academy]] clergy(1928–30). He also edited the religious journal ''Nyva'' (1922–32).<ref name=UKRAINIAN>Bohdan R. Bociurkiw. ''[http://www.encyclopediaofukraine.com/display.asp?AddButton=pages\K\O\KostelnykHavryil.htm Kostelnyk, Havryil].'' Encyclopedia of Ukraine, Vol. 2 (1989).</ref> In the late 1920s Gabriel emerged as a critic of the Vatican's Uniate policy and became the leading representative of the ‘Eastern’ (anti-[[Latinization]]) orientation among the [[Greek Catholic]] clergy.<ref name="UKRAINIAN"/>

In attempting a critical assessment of Father Gabriel's motives, [[Eastern Catholic Churches|Byzantine-Ruthenian Catholic]] priest Christopher Lawrence Zugger argues that his motivations were fuelled Father Gabriel was motivated partly from the hope of saving his son (who, he had been told was a prisoner of the Soviets), partly out of anti-[[Latinization|Latin Catholic]] feelings, and partly out of conviction.<ref>Rev. Christopher Lawrence Zugger. ''"[http://books.google.ca/books?id=HnUnJ7X10BMC&printsec=frontcover#v=onepage&q&f=false The Forgotten: Catholics of the Soviet empire from Lenin through Stalin]."'' Syracuse University Press, 2001. p.423.)</ref> The ''[http://www.encyclopediaofukraine.com/default.asp Encyclopedia of Ukraine]'' adds that Fr. Gabriel's theological position made him a target of NKVD pressure and blackmail during the 1939–41 Soviet occupation of Galicia, when the authorities first tried unsuccessfully to have him organize an ‘away from Rome’ schism in the Ukrainian Catholic church; and when the Soviets reoccupied Galicia in 1944 and arrested the entire Ukrainian Catholic episcopate, he was finally compelled to assume chairmanship of the Initiating Committee for the Reunification of the Greek Catholic Church with the Russian Orthodox Church.<ref name="UKRAINIAN"/>

In sum, this religious movement which started with the anti-[[Latinization]] orientation among the Greek Catholic clergy led by Fr. Gabriel, and culminated in the state-sponsored Synod of Lvov, came at the same time that the political atmosphere in the area changed, as the remnants of the Nazi regimes, various nationalistic groups, the Bolshevik forces, and religious differences all collided with the sincerity of the people. In that environment, many of the clergy and laypeople returning to Orthodoxy became victims of fanatics, both religious and political.

On [[September 20]], 1948, after the [[Divine Liturgy]] Fr. Gabriel was attacked on the steps of the [[w:Church of Transfiguration, Lviv|Cathedral of the Transfiguration in Lvov]] and killed by one, Vasily Pankiv, a terrorist who killed himself immediately after his deadly assault.  According to the official Soviet version, he Pankiv was a member of the terrorist group led by [[w:Roman Shukhevych|Roman Shukhevich]], chief of the Ukrainian Rebel Army (UPA).<ref name=Interfax>Interfax-Religion. ''[http://www.interfax-religion.com/?act=news&div=5186 The initiator of elimination of the Ukrainian Greek-Catholic Church could be canonized].'' 23 September 2008, 12:16.</ref><ref>{{ru icon}} Из мемуаров Петра Судоплатова. ''[http://militera.lib.ru/memo/russian/sudoplatov_pa/08.html «Военная Литература» Мемуары: Глава 8. «Холодная война» - Дорога к Ялте и начало мирного противостояния].''</ref> HoweverIn addition, representatives of the UPA denied any involvement an official bulletin printed in the murder. The ''[http://www.encyclopediaofukrainejmp.comru/default.asp Encyclopedia Journal of Ukrainethe Moscow Patriarchate]'' argues that although "Soviet authorities have blamed his murder on the Vatican (1948, № 10) and Ukrainian nationalists, the evidence suggests signed by Metropolitan Nicholas (Yarushevich) stated that the assassination Kostelnik was masterminded "killed by an agent of the Soviet policeVatican."<ref name="UKRAINIAN">{{ru icon}} ''[http://www.jmp.ru/ Журнал Московской Патриархии].'' 1948, № 10, стр. 9.</ref>

An official bulletin printed However, representatives of the UPA denied any involvement in the murder. And the ''[http://www.jmpencyclopediaofukraine.rucom/ Journal default.asp Encyclopedia of the Moscow PatriarchateUkraine]'' (1948states that although "Soviet authorities have blamed his murder on the Vatican and Ukrainian nationalists, № 10) and signed by Metropolitan Nicholas (Yarushevich) stated the evidence suggests that Kostelnik the assassination was "killed masterminded by an agent of the VaticanSoviet police."<ref>{{ru icon}} ''[http:name="UKRAINIAN"//www.jmp.ru/ Журнал Московской Патриархии].'' 1948, № 10, стр. 9.</ref>

Fr. Gabriel was buried in the [[w:Lychakiv Cemetery|Lychakov cemetery]] in Lvov. His funeral was attended, at least according to some Soviet authors, by around 40,000 people. Church leaders decided to inform the highest Soviet administrators of the 'great loss ' they suffered. Among others, J.V. Stalin and Nikita Khrushchev were contacted.<ref>[http://rdsa.tripod.com/biografeng.html Havriil Kostelnik (1886-1948)].</ref>