From 364- November 17, 375 Gratian shared the rule with his father and from 364-378, with his uncle, Valens.
When Valentinian I died at Brigetio [also in Pannonia], Gratian became senior Augustus in the West. He appointed his half-brother, Valentinian, co-ruler (Valentinian II).
Gratian fought against the raiding Alamanni and only came too late to help his uncle against the Goths at the Battle of Adrianople [see section He (Thrace) in Map of the Roman Empire in 395]. After Valens was killed, Gratian appointed Theodosius Augustus in the East, on January 19, 379, so that he could return to the West.
In 381 Gratian moved the capital from Trier to Milan.
In 383 Gratian went to Gaul to intercept Magnus Maximus who had been appointed emperor in Britain. Gratian's troops deserted him, so he fled to what is now Lyons and was then Lugdunum. Andragathius who was Maximus's magister equitum, killed him August 25, 383.
Christianity
In 379, Gratian recalled those bishops whom the Arian emperor Valens had expelled from the East. Because of his Christian beliefs, Gratian eliminated "Pontifex Maximus" from his imperial title, possibly in 379 or as late as 383, but in connection with receiving a senatorial embassy in Milan. This title had been held by all preceding emperors from the time of Augustus to Valentinian. He also refused the robe of the office. On the prompting of (saint) Ambrose, Galen removed the altar of Victory from the Roman forum.
Sources:
Encyclopedia Britannica
"On the Burial Places of the Valentinian Dynasty"
Mark J. Johnson
Historia: Zeitschrift für Alte Geschichte
Vol. 40, No. 4 (1991), pp. 501-506
"Gratian's Repudiation of the Pontifical Robe"
Alan Cameron
The Journal of Roman Studies
Vol. 58, Parts 1 and 2 (1968), pp. 96-102
