You do not get Gaal's key if you attack him. My first game I had a paladin, and managed to start a fight with the cultists via dialogue. The beholder appeared in the compound to defend the cultists, so I got the exp for killing it, but missed out on everything past the locked door. So yes--you miss a TON of exp by doing it the quick way.
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"Many are my names in many countries. Mithrandir among the Elves, Tharkun to the Dwarves; Olorin I was in my youth in the West that is forgotten, in the South Incanus, in the North Gandalf; to the East I go not"
--The Two Towers
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