Well, since they cast spells like a 14th level cleric, AND have lots of nifty powers, AC, DR, magic sword etc, I would say a level equivalent of about 24th, 25th...something like that?
After all, the CR3 unicorn counts as 8th level (+5 on the CR) because of its nifty movement and healing and stuff... The Ghaele is much better in every respect.
(n.b. it is curious though, is it not, that a 14th level cleric is CR14, while a Ghaele Eladrin has the spell casting ability of a 14th level cleric plus better hit points, saves, AC and abilities, and yet is one CR lower. Why? Is it because they "play fair" all the time, or don't like hurting anyone??)
Cheers