Lady Gaga is so unusual you could say, "what is Lady Gaga?" ;)

she is a singer,from america she has a very famous song which is name"Poker Face ",maybe you have listened it before http://baike.baidu.com/view/1928120.htm?fr=ala0_1_1 here is the details of her

Asking "What's Lady Gaga" implies that you want to know what she is rather than "who". So the better way to ask your question would be "Who is Lady Gaga". To answer the question though, she is a popular singer over here in the U.S.

hah oh my lady gaga

"who is Lady Gaga?" She's a pop-dance singer!