GERO (c. 900-965), margrave of the Saxon east mark, was probably a member of an influential Saxon family. In 937 he was entrusted by the German king Otto, afterwards the emperor Otto the Great, with the defence of the eastern frontier of Saxony against the Wends and other Slavonic tribes; a duty which he discharged with such ability and success that in a few years he extended the Saxon frontier almost to the Oder, and gained the chief credit for the suppression of a rising of the conquered peoples in a great victory on the 16th of October 955. In 963 he defeated the Lusatians, compelled the king of the Poles to recognize the supremacy of the German king, and extended the area of his mark so considerably that after his death it was partitioned into three, and later into five marks. Gero, who is said to have made a journey to Rome, died on the 20th of May 965, and was buried in the convent of Gernrode which he had founded on his Saxon estates. He is referred to by the historian Widukind as a _preses_, and is sometimes called the "great margrave." He has been accused of treachery and cruelty, is celebrated in song and story, and is mentioned as the "marcgrâve Gêre" in the _Nibelungenlied_. See Widukind, "Res gestae Saxonicae," in the _Monumenta Germaniae historica_. _Scriptores_, Band iii.; O. von Heinemann, _Markgraf Gero_ (Brunswick, 1860). GEROLSTEIN, a village and climatic health resort of Germany, in the Prussian Rhine Province, attractively situated on the Kyll, in the Eifel range, 1100 ft. above the sea, 58 m. W. of Andernach by rail, and at the junction of lines to Trèves and St Vith. The castle of Gerolstein, built in 1115 and now in ruins, affords a fine view of the Kyllthal. Gerolstein is celebrated for its lithia waters, which are largely exported. Pop. (1900) 1308.