Mount Gebra is located in Klatch near to the fortress called the Fist of Gebra. It is mentioned in Sourcery, as a mountain where goats graze. The Seriph of Al Khali compares Conina's hair to it. It also is mentioned in Jingo.
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Possibly a reflection of Mount Hebron in the Bible?
Since it is in Klatch which is modeled on an Arab country and "al" is the definitive article in Arabic perhaps Pratchett is playing on "The" Gebra or Algebra the branch of mathematics made possible by the invention of zero.