In American and Canadian English, the past participle of the verb get is usually gotten. For example, we might say, “I have gotten behind on my work,” or, “The book was not gotten easily.” Got is the participle in some uses, though, such as where has got to or have got to means must (e.g., “We have got to go to the store.”) and where has got or have got means has or have (e.g., “I have got five sisters.”)
In the main varieties of English from outside North America, the past participle of get in all its senses is usually got. Gotten appears occasionally, and it is standard in a few set phrases such as ill-gotten gains, but the shorter form prevails by a large margin.
That gotten is primarily used in North America has given rise to the mistaken belief that it is American in origin and hence new and inferior. But gotten is in fact an old form, predating the United States and Canada by several centuries. It fell out of favor in British English by the 18th century, but it was eventually picked up again on the other side of the Atlantic, perhaps by analogy with forgotten.
The vehemence of some Britons’ scorn for gotten likely has to do with the fact that it has gained ground in British English over the last couple of decades. Many English speakers from outside North America resist the encroachment of so-called Americanisms (many of which, like gotten, are not actually American in origin) on their versions of English, and, for mysterious reasons, some feel especially strongly about gotten.