#It's the Ring Verse.

This is almost certainly a fan image as MPF points out in the comments below. The Gondorians had no reason to display the Ring Verse on a banner, and almost certainly were unaware of it's existence in the end of the Third Age, with only Gandalf having read it while hunting for evidence of the One Ring.

> Ash nazg durbatulûk, ash nazg gimbatul, ash nazg thrakatulûk, agh burzum-ishi krimpatul

This is clear from the bottom right "quarter", bottom left "quarter" and top left "quarter" all beginning with the same words, "ash nazg".

It can be a bit difficult to read because the bottom half is actually upside down. I've attempted to provide a visual guide below:

[![Image of the white tree of Gondor with the Ring Verse in Tengwar][1]][1]

You can compare the above with the Ring Verse from the books, you'll notice that stylistically they are identical.

[![Ring Verse from Tolkien Gateway][2]][2]

For interested parties, I used [this resource](https://waymoot.org/home/tengwar/teng.html) to reverse transliterate the Tengwar characters into English, and [this resource](http://3rin.gs/tengwar) to check that my conversion was accurate by transliterating it from English back into Tengwar. Thankfully I did "Ash nazg" first and the rest was easy.

  [1]: https://i.sstatic.net/uRF9zm.gif
  [2]: https://i.sstatic.net/P6wcM.png