Gerlach was the son of Henry I of Isenburg-Grenzau and his wife, Irmingard of Büdingen, Countess of Cleeberg. Gerlach was a namesake of a senior Gerlach of Isenburg who served as Vogt of the Archbishopric of Trier from c. 1130 to 1147. After the death of his father, Henry I, in ca. 1227, Gerlach reigned jointly with his brother Henry II over the paternal inheritance. From 1247, Gerlach took the title of “Lord of Limburg.” On May 22, 1258 the division of the inheritance was finally sealed: Gerlach won the dominion of Limburg and Henry II the Countship of Isenburg-Grenzau as their sole possessions. From a trip to Italy in 1231, Gerlach brought monks of the Franciscan Order to whom he gave permission to construct a monastery in Limburg. This makes it one of the oldest Franciscan monasteries in Germany. The monastery was later endowed by the House of Limburg. As the Lord of Limburg, Gerlach joined the opponents of the Hohenstaufen. He was probably involved in the looting of Worms in 1243/44. After the Great Interregnum, Gerlach supported King Rudolph of Habsburg. In 1276, he was commissioned by the king to confirm the Counts of Diez in their imperial fief. In 1279, there was an uprising of the citizens of the town of Limburg, and Gerlach was expelled from the city. After negotiations, he was able to return to his castle in Limburg, but he had to grant the citizenry far-reaching freedoms. These freedoms included a pause in the collection of taxes and self-jurisdiction of marriages and housing. In the case of a dispute between the House of Limburg and the city, it would be decided by the jury of the City ofFrankfurt. Upon his return, Gerlach expanded Limburg Castle, building the residential tower which is still extant today. To secure his position, Gerlach sought dynastic connections with neighboring lordly houses, including Nassau, Westerburg, and Diez. His daughter Agnes married Henry I of Westerburg in 1267. Around the year 1270, his daughter Imagina married Count Adolf of Nassau. By Adolf of Nassau, Gerlach was appointed Burgmann of the Imperial Castle of Kalsmunt in Wetzlar in 1287. On June 5, 1288 Gerlach, together with Adolf of Nassau and Henry of Westerburg, participated in the Battle of Worringen. They supported Henry of Westerburg's brother, Siegfried, the Archbishop of Cologne. Siegfried’s defeat in the battle had no significant consequences for Gerlach. Gerlach I of Limburg died in 1289 during his participation in a campaign of King Rudolph in the Black Forest. He was succeeded by his son, John I.
Marriage and descendants
Gerlach married Imagina of Blieskastel. The couple had at least five children:
