Short answers, pulled from the story.
Gondor is Tolkien's Sindarin word for "Stone-land." The Rohirrim knew it as Stoningland, the same meaning in their own tongue. Researchers Wayne G. Hammond and Christina Scull have proposed the Quenya equivalent Ondonórë.
Gondor was co-founded by the brothers Isildur and Anárion, sons of Elendil of Númenor. They arrived in Middle-earth after the destruction of their island homeland and established the kingdom together, with Isildur building Minas Ithil and Anárion building Minas Anor.
Gondor's royal line died out after wars with the Wainriders and other conflicts during the Third Age ended the dynasty of Meneldil. A hereditary line of Stewards then governed in the absent monarch's name, carrying a white rod as their badge of office, never sitting on the high throne, and never being accepted as kings.
Scholar Sandra Ballif Straubhaar identifies ancient Rome as the closest parallel: both Gondor and Rome were founded by two brothers, both descended from an earlier destroyed civilization, and both endured centuries of decadence and decline. Classical scholar Miryam Librán-Moreno offers a detailed Byzantine comparison, noting that both Gondor and the Byzantine Empire faced powerful eastern and southern enemies and both endured a final major siege from the east, though Minas Tirith survived while Constantinople did not.
Peter Jackson's team modelled the structure of Minas Tirith on Mont Saint-Michel, the inhabited tidal island and abbey in France. The production also incorporated Byzantine domes and Byzantine-styled civilian clothing, reflecting scholarly comparisons between Gondor and the Byzantine Empire. Film critic Roger Ebert called the result a "spectacular achievement" and compared it to the Emerald City from The Wizard of Oz.
Tolkien's earliest thoughts about Gondor appear in mid-1930s sketches for the legend of Númenor. The appendices to The Lord of the Rings were brought to a finished state in 1953-54. The final development of Gondor's history and geography took place around 1970, in the last years of Tolkien's life, with fuller narratives including the story of Isildur's death and the battles with the Wainriders published posthumously in Unfinished Tales.