As stated above, carbon steel that rusts away if the fish breaks the line.
For Bridge Gaffs, large Stainless might be used, but galvanized carbon steel is used most often.
Kept really sharp.
Fishing the Keys bridges, when a big edible is hooked and pulled to the surface, we clip a large treble hook attached to a Cuban YoYo, with thin rope line, on the 20lb mono, send it down to hook dinner, and pull the fish up.
When the surface is full of shrimp, and a Tarpon gets hooked, we snap or cut the line. Heavy weights are used to get down quickly to avoid the Tarpon, and get the Snappers and Snook.
Pic of Cuban YoYo with magic musical hook and fife fish call.
Duck calls don't raise them but a Taylor Swift tune.....
and it's Catch 'em Up.
When we were kids fishing in Willow Lake, Queens NYC, my buddy walked behind me as I was casting.
Caught him in the ear lobe.
As the Doctor was trying to pull it out, unsuccessfully,
I suggested he cut the barb off first.