According to Robert Pitcairn, an early historian of Clan Kennedy, Earl Gilbert was a "werry greidy manne and cairitt nocht how he gatt land sa that he culd cum be the samin". During the era of the Protestant Reformation, Earl Gilbert desired the Abbey of Glenluce and entered into transactions with the abbot in an effort to hold the abbey by feudal tenure. However, before the transaction could be completed, the abbot died. Undeterred, Gilbert turned to a monk in the abbey, requesting him to "counterfeit" the abbot's handwriting to draw up a deed and to forge the signatures of all the members of the convent. This ruse succeeded, but, fearing the monk would reveal this deceit, he solicited a peasant "to stik him". That done, he began to fear that the peasant would reveal the crime and arranged with his uncle, Hugh of Bargany, to accuse the peasant of theft and have him hanged. "And sa the landis of Glenluse wes conqueist."^[3]

In 1565, he seized Allan Stewart, the Commendator of Crossraguel, and imprisoned him at Dunure Castle, seeking to obtain from him certain of the rights over the lands of Crossraguel Abbey. For two days he was left to consider his fate; when Stewart proved recalcitrant, Cassilis had him dragged to the Black Vault of Dunure, and roasted him alive over a fire until he was willing to subscribe to the charters the Earl had drawn up.^[2] Stewart was finally rescued by his brother-in-law, the Laird of Bargany, who captured Dunure and procured his deliverance.^[4] The rescue, however, occasioned a feud between the subsequent Earls of Cassilis and Lairds of Bargany.