Gilbert de Clare changed sides

On May 14, 1264 Simon de Montfort and his ally, Gilbert de Clare, Earl of Gloucester, led the barons to victory at Lewes.
They captured King Henry III, his son Edward and his brother Richard.
In May 1265 Prince Edward escaped from de Montfort's custody. Gloucester changed sides and joined Edward.
Suppose Gilbert de Clare, Earl of Gloucester stayed an ally to Simon de Montfort. What happens then?
 
Gilbert de Clare would not have shared Prince Edward's victory at Kenilworth on July 16, 1265.
Gilbert de Clare would not have shared Prince Edward's victory in the Battle of Evesham on August 4, 1265.
In the Battle of Evesham, Simon de Montfort was slain.
Would the Earl of Gloucester also have been killed?
 
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