I'm pleased to hear that you have had success in obtaining records.
In my experience, Armed Guard crew lists include primarily seamen (1st class or 2nd class), all of whom can be assumed to have served as gunners. "Gunner" is not a U.S. Navy rank (technically not rank in the case of an enlisted sailor but rating instead). Additionally there would have been one or more gunners mates, which is a rating, and which indicates a petty officer (non-commissioned officer). There would often be one or two signalmen, sometimes a radioman, sometimes some other rating(s). I wouldn't be surprised if the seamen/gunners would have been cross-trained among different weapons but signalmen and radiomen were unique ratings with additional training required before shipboard assignment. Additionally there would be one officer, generally an ensign, lieutenant junior grade or at most full lieutenant.
Seamen/gunners were generally trained for and assigned to a particular type of weapon: 3" gun in the bow, 5" gun in the stern (inches indicating the diameter of the shells the guns fired), and 20 mm. machine guns, located at various places about the ship. These were typically the defensive weapons found on Liberty ships. The 20 mm. machine guns were antiaircraft weapons, while the 3" and 5" guns were designated "dual purpose," meaning they could be used against aerial or surface targets. So your grandfather, a seaman 1st class, was almost certainly a gunner and indeed an antiaircraft gunner, but his primary assignment might have been on any of at least three types of weapons.
I have checked back through the message board and I see that you and I first communicated in November 2021, barely five months ago. So you must have had rather good luck in obtaining information from the National Archives if you have received Armed Guard logs. How long did it actually take to receive information? Would you mind indicating the cost (cost per page and/or total cost)? Such up-to-date information would be useful for me in responding to other inquiries similar to yours.
If you have additional questions, you know where to find me.
Ron Carlson, Webmaster
Armed Guard / Merchant Marine website