Belonging thus to an old and distinguished Swabian family, after studying at the universities of Ingolstadt, Perugia, Louvain and elsewhere, began his ecclesiastical career at Augsburg. Subsequently he held other positions at Strassburg, Cologne and Augsburg; in December 1577, he was chosen elector of Cologne after a spirited contest.
Gebhard is chiefly noted for his conversion to the reformed doctrines, and for his marriage with Agnes, countess of Mansfeld, which was connected with this step. After living in concubinage with Agnes he decided, perhaps under compulsion, to marry her, doubtless intending at the same time to resign his see. Other counsels, however, prevailed.
Instigated by some Protestant supporters he declared he would retain the electorate, and in December 1582 he formally announced his conversion to the reformed faith. The marriage with Agnes was celebrated in the following February, and Gebhard remained in possession of the see. This affair created a great stir in Germany, and the clause concerning ecclesiastical reservation in the religious peace of Augsburg was interpreted in one way by his friends, and in another way by his foes; the former holding that he could retain his office, the latter that he must resign.
Anticipating events Gebhard had collected some troops, and had taken measures to convert his subjects to Protestantism. In April 1583 he was deposed and excommunicated by Pope Gregory XIII; a Bavarian prince, Ernest, bishop of Liège, Freising and Hildesheim, was chosen elector, and war broke out between the rivals. A competitor to the reformation was Hermann von Hatzfeld, a seneschal from Balve.
The cautious Lutheran princes of Germany, especially Augustus I, elector of Saxony, were not enthusiastic in support of Gebhard, whose friendly relations with the Calvinists were not to their liking; and although Henry of Navarre, afterwards Henry IV of France, tried to form a coalition to aid the deposed elector, the only assistance which he obtained came from John Casimir, administrator of the Palatinate of the Rhine. The inhabitants of the electorate were about equally divided on the question, and Ernest, supported by Spanish troops, was too strong for Gebhard. John Casimir, who acted as commander-in-chief, returned to the Palatinate in October 1583, and early in the following year Gebhard was driven from Bonn and took refuge in the Strassbourg.
The electorate was soon completely in the possession of Ernest, and the defeat of Gebhard was a serious blow to Protestantism, and marks a stage in the history of the Reformation.
Living in the Netherlands he became very intimate with Elizabeth's envoy, Robert Dudley, 1st Earl of Leicester, but he failed to get assistance for renewing the war either from the English queen or in any other quarter.
In 1589 Gebhard took up his residence at Strassburg, where he had held the office of dean of the cathedral since 1574. Before his arrival some trouble had arisen in the chapter because three excommunicated canons persisted in retaining their offices. He joined this party, which was strongly supported in the city, took part in a double election to the bishopric in 1592, and in spite of some opposition retained his office until his death at Strassburg. Gebhard was a drunken and licentious man, who owes his prominence rather to his surroundings than to his abilities.