To gaga something is to rip off, steal,
or copy someone else's idea or product and try to pass it off as your original idea. (example 1)
Can also be used as a confession when one is willing to admit they got the idea elsewhere and does not wish to take credit from the inventor (example 2)
Person 1: "Stephanie's homemade purse is so cute! How creative!
Person 2: "she totally gagaed that off Pintrest..."
Person 1: "You made that! That is so ingenious!"
Person 2: "Oh I gagaed this from some girls blog. It has instructions and everything on how to make your own."