Gamil Zirak was the master of Telchar of Nogrod, one of the greatest of all Dwarf-smiths. We can presume that Gamil was also a master smith in his day, and passed on all of his knowledge to Telchar, who perhaps was then able to push the craft even further. The only reference is in The Unfinished Tales, where he is called "Gamil Zirak the old".
For some years now, the presumption has been that Gamil translates as "old". This is based on the reference "Gamil Zirak the old", and that Zirak shows up in Zirak-zigil. At the time Tolkien wrote about Gamil Zirak, he may have still interpreted zirak as "silver", before he changed zigil to "silver, the color" and thus zirak to "tine, spike, point". Because of that change, we will assume that here Zirak means the same.
In the past, I have resisted the idea that Gamil translated as "old". Now, I think it is likely to at least be something similar since, as I noted in the section on Azaghâl, names of Dwarves have similarities to names in Semitic languages like Arabic, including the laqab. If the Dwarf names of the First Age are descriptors like "Oakenshield" is for Thorin, then I can see the possibility of gamil being "old".
Working with this assumption that gamil translates to something related to "old", we can observe that the vocalization CaCiC is the same seen in gabil. Both then may be from verbal roots, so G-M-L would be "to age, to grow old". Gamil (or possibly gamîl) would then be a verbal adjective meaning "old, aged". Combined with zirak, the two could be a type of construct phrase known in Arabic as the "construct of qualification". Essentially, this is where an adjective is placed in construct with a noun that follows it which qualifies the adjective. An example would be "pure of heart". That is probably the case here, and so gamil is in the composition form, singular, and nominative. If so, it would translate "old/aged of the tine/spike". A better way of translating into English might be "peak age". This would perhaps indicate how old Gamil Zirak was in relation to other Dwarves; he was at the very tine of the mountain in terms of age, so to speak. He may have already been quite mature when he began mentoring Telchar, and eventually reached an age that few Dwarves ever attain. He was indeed Gamil Zirak the old!
Gamil then is probably "aging, old" from a verbal root G-M-L "to age, grow old" and is singular, nominative, composition.
Zirak is "tine, spike" and is singular, nominative, indefinite. See the entry on Zirak-zigil for more information.
The Unfinished Tales, pg 80 (or 76, depending on version)