I don't believe Gandalf would utter anything other than "You shall not pass!" It is formal, and it is almost biblical in intonation (THOU SHALT NOT!).
From a different angle, he is speaking to a fellow Maia, which is rather amusing: why would he be speaking in Westron?
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And your little sister's immaculate virginity wings away on the bony shoulders of a young horse named George who stole surreptitiously into her geography revision.
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