#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:
You can compare the above with the Ring Verse from the books, you'll notice that stylistically they are identical.
For interested parties, I used this resource to reverse transliterate the Tengwar characters into English, and this resource 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.