A history of Lanthony Abbey records that “Hugo de Laci” had “nepos...miles” who, after a long military career, eventually became a Templar (“sub habitu Templariorum...”). His co-identity with the unnamed “nepos” of Hugh de Lacy is indicated by the charter dated May 1160, which records a treaty between Louis VII King of France and King Henry II, subscribed by "…Willelmo Pavet magistro Templi et fratribus...Gilberto de Laceio…". The precise date when Gilbert joined the Knights Templar has not been ascertained. Gilbert inherited the share of the family’s Norman fiefs held by Walter de Lacy’s branch of the family, as shown by a list of fiefs of Bayeux church, dated to , which records “feodum de Lacey in Campellis” held by “duorum militum...Guilleberti et Henrici”.
...The following charter suggests that Gilbert at first supported King Stephen after the death of Henry I: "...Gilberti de Lacy baronis..." witnessed the charter dated Easter 1136 under which King Stephen granted property to Winchester Cathedral. However, Eyton highlights another charter at the same time with a similar witness list which includes “Ilbert de Lacy baron” (presumably the charter dated Easter 1136 for Bath abbey), and suggests that the former document represents a transcription error. It should be noted that Ilbert is named in numerous documents of King Stephen around the same time. Gilbert’s support for Empress Matilda’s claim to the throne in England is indicated by two documents: firstly, the Gesta Stephani Regis records that "Galfridus…Taleboth cognatus…Gislebertus de Laceio" was captured by King Stephen’s forces, dated to [1139/40], and secondly "...Gisleberto de Lasceio" witnessed the charter dated 25 Jul 1141 under which Empress Matilda created Miles Earl of Hereford. Some time afterwards, Gilbert switched sides and supported King Stephen: "Robertus regis filius Gloecestriæ consul" notified an agreement with the bishop of Bayeux relating to fiefs claimed by the latter, including “tota feoda Ilberti et Gilberti de Laceio...apud Laceium et apud Campels”, which were held “de Baiocensi ecclesia et de episcopo”, by charter dated Sep 1146. Rivalry between Gilbert de Lacy and the heirs of his [presumed cousin] Sibylla de Lacy...concerning the succession to the Lacy estates in England is indicated by an agreement dated to [1147/49] between Roger Earl of Hereford (husband of the older daughter of Sibylla de Lacy...and Matilda’s supporter) and William Earl of Gloucester to combine forces against Gilbert de Lacy. Presumably following some activity implementing this agreement, Gilbert de Lacy complained to Gilbert Foliot Bishop of Hereford that the Earl of Hereford had captured one of his knights “Ricardum...Britonem” and was holding him to ransom. "Gilbertus de Laci et Robertus filius eius" confirmed the donation of a tenement in Droitwich to the monks of Worcester Cathedral priory by charter dated to . After King Stephen made peace with the future King Henry II, Gilbert joined the latter: "...Gisl[eberto] de Laceio..." witnessed the charter dated to [1153/Apr 1154] under which Henry Duke of Normandy confirmed the foundation of Biddlesden abbey, and "...Gilbertus de Lacy..." witnessed the charter dated 1154 under which Duke Henry confirmed Stanton Harcourt to “Milicenti uxori Ricardi de Camvilla”. The 1157 Pipe Roll records "Gillto de Lasci" pardoned for sums in Herefordshire, Gloucestershire and Shropshire. He became a Knight Templar between 1157 and May 1160: "…Willelmo Pavet magistro Templi et fratribus...Gilberto de Laceio…" subscribed the charter dated May 1160 which records a treaty between Louis VII King of France and King Henry II. A listing of Templar properties dated 1185 includes land “apud Guttingres…Holeford” donated by "Gileberti de Laci".
Eyton conjectures that Gilbert de Lacy died, or at least retired from the world, between 1158 and 1163. The mention in the 1159/60 Pipe Roll of his son Robert suggests that Gilbert had already relinquished his estates in his son’s favour. 1