Gammon is usually served warm as it is bought uncooked. Ham is usually left to cool after cooking and served cold. But I know of no rule saying that you can't warm it.
When we have a gammon joint, I serve it hot on Sunday, cold on Monday and made into a carbonara sauce on Tuesday. If it's big enough, we might also have cold gammon sandwiches for lunch on Wednesday.
I wouldn't warm up slices of gammon in the microwave. I wouldn't enjoy that at all.