I think
Gil-galash is an error for
Gilgamesh. I only know of
Gil-galash from a dubious reference at
Small Gods
There’s also the Biblical placename
Gilgal where Saul was crowned.
There is also the region of
Galaad mentioned in the Bible in the Latin Vulgate (which the King James version and later Protestant translations usually render as
Gilead.) The name appears as a personal name in the later Grail stories, rendered as
Galahad in English in Thomas Malory’s
Le Morte Darthur.
You can find parallels to almost all of
Tolkien’s names and words in some language or other, but usually no evidence that they played in part in Tolkien’s mind when he coined the word or phrase and very seldom will the meaning in the other language have any signficance in respect to Tolkien’s use.
Tolkien’s earliest surviving work ion
Elvish languages do contain words and names that do seem to be based on obvious sources. But as he revised his
Elvish tongues such apparently obvious sources grew less and less.
So one can usually never know whether because one of Tolkien’s words or names echoes a real name or real word in another tongue in the reader’s ear whether in fact there was any influence.
Tolkien himself might not have known. In many cases Tolkien developed his forms from previously created stems following rules he had worked out for the creation of different grammatical forms in his
languages.