Hi Sarcandra Gibber is in my dictionary (to speak rapidly and unintelligibly, typically through fear or shock : they shrieked and gibbered as flames surrounded them ) and I've heard it used and seen it written throughout my life. I'm certain it's not a "new" word, and equally certain that it's not commonly used. So, your Professor is incorrect about the word being unacceptable, but correct about being uncommon.
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