The act of taking something normal or good and making it very, very strange.
History: The phrase originated from Lady Gaga, who was overtaken by her own success, and as a result, went from being cool and new, to just weird and annoying.
Other uses of the phrase: To GaGa, GaGafy, GaGa It, GaGa'd
Examples of of GaGaing:
Jon: Hey, did you see the video for Telephone?
Geoff: Ugh, unfortunately... Man, she totally GaGa'd it...