is the Dwarves' name for a portion of the river Gelion (in eastern Beleriand) that was south of the junction with the Asgar river.  It was after that point that it became increasingly large and swift due to several more tributaries.  The only translation given is "great river".

The word can be broken into gabil + -ân.  Compare Gabilân with Gabil-gathol "Great Fortress" (below).  Both have the concept of "great", and the common element between them is gabil.  The root G-B-L might actually be verbal in nature, rather than noun/adjectival.  Arabic, Hebrew, and other Semitic languages have "stative verbs".  In a sentence where these verbs are in the perfect or imperfect form (the two primary aspects for most Semitic languages), they can express that the subject has been in that state or is becoming that state.  What makes this more likely is that Arabic stative verbs then have an adjectival form, usually CaCîC, which says that the subject is currently in that state when used as a predicate.  Here, the verbal adjective "great" should be in composition form since it has a derivational suffix -ân attached.  Note that the stem has not been reduced to a CVCC form here, unlike Nargûn and Tharkûn.  That may show that the vowel of the second syllable is stressed, making it less likely to be reduced, and the long vowel makes this even more probable.

The second element, -ân, is a bit more tricky.  It could mean "river" if directly taken from the translation.  The root would be something like '-N-N and use the same vocalization as bark "axe", giving us 'ann, and from there shifting to 'ân.  Roots with geminate radicals that use a vowel pattern like CVCC often
drop the final consonant and lengthen the vowel to compensate.

I think, however, that there is another explanation.  That is, -ân may be a suffix, and if so, it looks much like the -ûn of Nargûn and Tharkûn, plus the -în of Nuluk-khizdîn.  Tolkien originally wrote Nuluk-khizdîn as Nuluk-khizidûn.  Notice the -ûn.  I think it shows that the vocalization was changed, but the meaning was probably held intact.  There may be a morphological or phonological reason for it.  Does the CiCiC pattern of khizid- influence the ending somehow?  I doubt it, since /d/ is not a weak consonant.  I could also see it being related to gender, number, the type of root (noun, adjective, or verb), or other factors.  However, any of those are difficult to analyze, much less prove.
Nargûn and Tharkûn, for example, are a geographic place (Mordor) and a personal name (Gandalf), but they both have the -ûn ending.  It's more probable, in my view, that Tolkien simply liked the look and sound of khizdîn better, which seems to be the manner in which he often worked.  Decide on a word form, and then figure out what the etymology was.  Fortunately, with Khuzdul this similar to how Semitic languages work, in that two words may use slightly different vocalizations, but the basic morphological structure is still intact.  I see this as being a sort of "suffix of specificity or unity", the same as in Nargûn, Tharkûn, and Nuluk-khizdîn, with the vowel simply being variable.  Probably the best translation for this suffix is either "one" or whatever type of entity it is; in this case either "Great One" or "Great River".

Gabilân is probably "Great (One)" and singular, nominative, definite (proper noun).

The War of the Jewels, pg 336