Gammon and pineapple is the cured-ham-steak sandwich with a sweet acid set deliberately against the salt, and the pineapple is doing chemistry, not decoration. A gammon steak is dense and heavily cured, and the single most useful thing you can put against that wall of salt is something both sweet and sharp. Pineapple is exactly that: its sugar reads against the savoury cure and its fruit acid cuts the fat at the same time, so a ring of it on a gammon steak does two jobs in one move. The pairing is a pub-counter classic for a reason, and in sandwich form the defining fact is that the pineapple is the counterweight the salted steak is built around, not a garnish laid on at the end.
The craft is balancing the fruit against the meat and keeping it from wetting the bread. The gammon is grilled or fried until the fat catches and cut so it is not a single tough plank between soft slices. The pineapple, a ring or a crushed layer, is drained well, because the juice that makes it useful as a flavour is also what soaks a sandwich into collapse if it is let loose against the crumb. Often the fruit is griddled alongside the steak, which drives off some of that water, concentrates the sugar, and gives it a caramelised edge that stands up better between bread than a cold wet slice. Soft plain bread carries the load and butter on it is structural, sealing the crumb against the fruit and bridging the salt of the cure across to the wheat so the sandwich reads as balanced rather than as salt and sugar shouting past each other.
The variations are the same steak with the counter swapped for another. A fried egg replaces the sweet-acid cut with a soft yolk that coats rather than slices through the salt; mustard or a sharp pickle does the cutting dry, with no sweetness at all; the plain gammon sandwich is the base with the salt left to stand alone. Each is the cured steak meeting a different answer to the same problem, and they deserve their own articles rather than being crowded in here.