Not something you fish with, but part of your uniform. It is the alignment of your buttons, belt buckle, and your fly. While in dress uniform these should form a nice straight line. You are supposed to also have this same arrangement in your working uniform, but that is not always possible. This is just another one of those pay attention to details the Navy is famous for.

In boot camp everyone is so worried they will mess up their uniform so they are always checking to make sure their gig line is straight. I’ll even admit to ding this after I got out of the Navy for a few weeks. I would go to work and from time to time check to make sure things were squared away.

One of the pranks we would pull on the new guys on ship was to send them looking for gig line. The 4th class Midshipmen were so good at looking high and low for it. 99% of the time they ended up being sent to the bridge to ask the OOD for it.