"You have gotten good at something" or "you have become good at something" means that you weren't originally good at it, but you trained or worked hard to become good. Eg. "You have gotten really good at basketball, all that training paid off!" "You are good at something" is just a general statement whether you were born with talent or you worked hard to become good. Eg. "You are very good at the piano" I guess the difference is that you would only use the first sentence, if you knew that the person wasn't good before. To compliment someone for their strengths, maybe just use the second.