Yes, Gondor is Byzantium, namely the Roman empire surviving far into the middle ages. The parallels to history in Middle Earth are manifold, striking, and intentional. You will likewise see an ever-churning westward movement of migrating tribes such as Wainsriders, just as these kind of movements characterized Central Asia ever since the Huns and the first rise of the Turkic tribes up to the rise of Muscovy, with easterly tribes displacing Indo-European peoples as they went (Tajiks are Iranian Central Asians, and were always viewed as softer, richer city dwellers, too sophisticated for their own good by the warlike Turkic nomads).
Furthermore, the battle of Minas Tirith is based both upon the second siege of Vienna (with Rohirrim riders for Polish hussars) and upon the fall of Constantinople, with the hopelessly outnumbered Gondorians being the last depopulated remnants of a once vast and sprawling empire. Since the Lotr trilogy isn't grimdark Silmarillion, good wins the day when the cavalry arrive. In
Silmarillion, things go to hell in a handbasket at the critical moment.Likewise, Gondor was critically crippled by a plague, just as the Roman empire was twice: First the Antonine plague that severely weakened the classical Roman civilization, and later on the Justinian plague that kicked the legs out from under the blossoming Eastern Roman world.
Also,
Mordor is modelled on the only country on Earth that is surrounded by mountains: Iran. With Gondor-Arnor versus Mordor standing in for the extremely long-running conflict between Persia and Rome.
It should be noted that the historical parallells don't stretch all the way. In Lotr there are no obvious equivalent to the Slavic ravages and settling of Pannonia, Thrace and Illyria, which saw northern tribes descend upon the Latin-speaking provincial population and virtually wipe it out, bar a few pockets remaining particularly in fortified coastal cities that maintained lines of communication to Constantinople (see
Vlachs). In reality, the precarious situation of the Eastern Roman Empire was even more pressed from all quarters than you get the impression of Gondor being. If Tolkien had followed history closer, the Rohirrim and Dunlendings would have swept and ravaged what is now Rohan, and peopled it in fierce competition to Minas Tirith with all her holdings. There would have been
wars upon wars between Rohan and Gondor, but this northern and western front is quiet in Middle Earth, unlike the real world where no quarter whatsoever was safe for the Byzantine realm.
If you find fantasy worlds vicious, then check out reality.