Gainas began his military career as a common foot-soldier, but later commanded the barbarian contingent of Theodosius' army against the usurper Eugenius in 394. Under the command of Gainas, a man of "no lineage", was the young Alaric of the Balti dynasty. In 395, he combined his forces with those of Stilicho and Eutropius to bring about the fall of Rufinus.
In 399 he replaced magister militum Leo after the latter failed to put down invasions led by Ostrogothic chieftain Tribigild, who was devastating Asia Minor. Gainas too failed to put down the invasions, although he blamed his failure on Eastern Roman Emperor Arcadius' palace chamberlain (cubicularius) Eutropius. Gainas then proceeded to install his forces in Constantinople, where he ruled for several months. He attempted in effect to copy the success of Stilicho in the West and posed a danger to the survival of the Eastern Roman Empire. He deposed all the anti-Goth officials and had Eutropius executed, though after the intervention of St. John Chrysostom the others were spared.
While a somewhat competent military commander, the zealous Arian Gainas was patently unable to administer a city of 200-400,000 whose Graeco-Roman populace intensely resented barbarian Goths and Arian Christians. Gainas' compromises with Tribigild led to rumors that he had colluded with Tribigild, his kinsman; when he returned to Constantinople in 400, riots broke out. He attempted to evacuate his soldiers but even then the citizens of Constantinople managed to trap and kill 7,000 armed Goths, spurred to action by the Empress Aelia Eudoxia.
In response, Gainas and his forces attempted to flee back across the Hellespont, but their rag-tag ad hoc fleet was met and destroyed by another Goth in Imperial service, Fravitta, who was subsequently made consul for 401 but was later accused of treason and executed as well. After this battle, Gainas fled across the Danube and was caught by the Huns under Uldin. Gainas was killed, and his head was sent by Uldin to Arcadius c. 400 as a diplomatic gift.
Herwig Wolfram, the historian of the Goths, notes that the death of Gainas marks an end to the relatively pluralistic Gothic tribal development with independent warbands: "thereafter only two ethnogeneses were possible: that of the Roman Goths within the empire and that of the Hunnic Goths at its doorstep".
- Herwig Wolfram, History of the Goths (1979) 1988:138; Wolfram's summary of the career of Gainas and Tribigild: pp 148-50.
- Friell, J. G. P.; Williams, Stephen Joseph (1999). The Rome that did not fall: the survival of the East in the fifth century. New York: Routledge. pp. 11–12. ISBN 0-415-15403-0.
- The kinship of Gainas and Tribigild is inferred by Wolfram (1988:435 note 182) from Sozomen VIII..4.2, with the understanding that Sozomen may have meant merely that they were of the same tribe.
- Wolfram (1979) 1988:135.
- Zosimus Book V 
- Socrates of Constantinople Book VI 
- Sozomen Book VIII 
- Theodoret Book V 
- George of Alexandria, Life of St. Chrysostom, in Photios, Myriobiblon, 96 
- Alan Cameron and Jacqueline Long, Barbarian and Politics at the Court of Arcadius, Berkeley et Los Angeles, 1993.
- Alexander Kazhdan (éd.), The Oxford Dictionary of Byzantium, 3 vols., Oxford University Press, 1991 (ISBN 0-19-504652-8)
- (fr) André Piganiol, L'Empire chrétien, PUF, Paris, 1972.
- Herwig Wolfram, History of the Goths (1978) tr., 1988.