Greek Catholicism is not really Catholicism if you know what I mean. Many Greek Catholic would rather switch to eastern Orthodoxy than Catholicism, which was the case in Belarus and Ukraine. Greek Catholicism came into existence after Brest Union (1596) to get Orthodox populations of Ukraine and Belarus into jurisdiction of Vatican. Greek Catholic kept eastern Orthodox religious traditions including the important celebrations following Julian calender. This was the compromise. 75% of Belarusian ancestors were Greek Catholic at the last partition of the Polish-Lithuanian Commonwealth in 1795. During that time Russian impress Catherine II decided to bring all Greek Catholic of Belarus to eastern Orthdoxy and people accepted it voluntarily. By 1840 there were no Greek Catholic diocese remained in the Russian empire except possibly for Chelm which is in modern eastern Poland right on the border with Ukraine. There were few regions such as Transcarpathia in western Ukrainei and south-eastern Poland in which Greek Catholicism survived because those regions were incorporated into Austro-Hungary rather than Russian Empire.
Anyway, most Greek Catholic Ruthenians and Ukrainians follow Julian calendar in Ukraine today.