I have seen many Americans use this phrase "I have gotten" rather than "I have got",which is more frequently used in other english-speaking countries and although I have searched in many dictionaries I still have not understood when one should use this phrase.
There's a difference between the way the verb "to get" is conjugated in British English and American English. In the U.S., "gotten" is commonly used as the past participle for "get." So whereas in the U.K. you might say "I have got," in the U.S. you'd more often say "I have gotten" (or, "I've gotten").
"I have gotten bad grades on many tests in the past."
"I would have gotten candy from the store, but I didn't have enough money."
BUT NOT: "I've gotten some candy in my pocket." In cases where you're using "have got" as a synonym for "have" or "must," you wouldn't use the past participle conjugation. So, "I've got some candy in my pocket" and "I've got to go."
That last bit got a little confusing (might have gotten a little confusing!). But there it is. Gotten is the past participle of get in American English.