This is one of the two Dwarf cities of the Ered Luin in the First Age. In Sindarin it was called Belegost, and known in the language of Men as Mickleburg. It is translated as "Great Fortress", which is the same as both Belegost and Mickleburg. Mickle decends from Old English, meaning "great, abundant", and Burg is "castle" in Germanic languages.
Gabil is the element "great", as seen above in the explanation of Gabilân. As noted there, it is most likely a verbal root meaning "great, large, mighty in size". Here, it is in composition form as the first word in the compound, and is probably nominative and singular. The indefinite is probably also gabil, as we see the same pattern CaCiC in Gamil Zirak. That could be a construct phrase, so the possibility stills exists that it could be something like gabîl. For the most part, in the Hebrew construct state, only /â/ is reduced in a final syllable, but there is some hypothetical evidence that might not be true in Khuzdul. Tumunzahar and Buzundush may be reduced from Tumûn + zahar and Buzûn + dush. The first element of Azanulbizar was initially Azanûl, although we don't know if Tolkien still considered that a "valid" word form. Khuzdul is inconsistently spelled Khuzdûl, but Fundinul is always written with the short /u/. We can theorize that all long vowels in a final syllable are reduced in the composition from, and that the -ûl can also optionally be -ul or it just gets reduced in the composition form and when used as an attributive modifier to another word.
In any event, we will say gabil is "great, large, mighty in size" and a verbal adjective, singular, nominative, composition.
Gathol is pretty straightforward. It means "fortress", and should be singular, nominative, indefinite.
The full compound Gabil-gathol is thus "Great Fortress" and has adjective-noun word order.
The Silmarillion, ch 10
The War of the Jewels, pg 201, 209
The Lost Road, pg 274