"'Gotten'?? Isn't the Reg from Blighty?"
Ah, therein lies a tale.
Gotten is an older English word, current in the 18th Century. When the War of Independence came along the former colonies started to diverge in terms of the use of the language. Thank Noah Webster for a lot of that.
The short version is: "Gotten" carried on being used in the US but fell out of fashion in the UK.
The phrase "ill-gotten gains" is a remnant of its use here.